You should totally scale the single-monster encounters to make them memorable for everyone. You have multiple options for that.

Lair actions

If your single monsters are bosses or the like, you can give them lair actions. Just look at examples like the dragon ones (starting at MM 86). Rambling terrain, furious waterfalls, tempest winds, make your choice.

Modify your monsters

Read the chapter Modifying a Monster in the DMG 273. You can adjust simple things like giving immunity to a certain type of damage to the monster, give it a flying distance, give it better weapons (maybe even magical lootable ones!), or give it additional features (pick one from the DMG 280!).

Making a bigger version of a monster can also be a thing (like modify a medium monster into a large one). This will increase its hit points and also allow it to do more damage using weapons (x2)! (see Step 11. Damage DMG 277-278)

You can then re-calculate the challenge rating of your monster and make sure it is appropriate for your party !

Add minions

Powerful monsters don't always act alone. They often have hordes of minions at their command, and even bodyguards or lieutenants! If you don't want to add too many monsters to encounters, as you mentioned, you could give a try to the latter options! Simply add one or two less/much less powerful monsters to the fight, and this should rise the overall challenge rating of your encounters!

@Yotus already brought up some fine points, but one that I was surprised not to see was:

Ações lendárias

This is an existing mechanic that can be easily adapted to your specific monster. Take a look at dragons, vampires, liches, and just about any CR >15 monster, and you'll find legendary actions. These are actions a monster can do out of turn. They're usually fairly basic. Some examples are:

• move (usually with a free disengage on that movement)

• make a single melee attack

• do a single target/AoE debuff/damage effect (usually costs multiple legendary actions)

you can think of these as monster reactions, except they don't need triggers, and you are limited to 3 per turn rather than 1, and the monster still gets their reaction for an opportunity attack/counter spell/whatever.

These extra actions can help them gain ground on the squishier ranged characters/casters, lock down their heavy hitting opponents while attacking the support characters, or do some extra damage between turns. It almost makes it like there are 2 opponents there.

As a bonus, because they are at will, you can use them to control the fight a bit. If the fight is going to easy, you can 'forget' that your boss has legendary actions, and odds are your players won't remind you. If it starts swinging back their way too much, make a quick change, and your guy now gets 4 legendary actions on his turn, rather than just 3. Sure, its a bit railroad-y, but it will help you balance the fight on the fly.

Roll your boss into Initiative twice

Isto não é tecnicamente an existing mechanic, but its really quite simple. It works best for your skirmisher types: the quick agile boss, rather than the big lumbering one.

Just roll initiative for your boss twice, and put him in the turn order twice. He gets 2 turns, and two reactions, but otherwise works just the same way. Mechanically he is 2 people who have the same AC, have the same movement speed, always stay in the same square together, and have a shared health pool (like a 2-way vínculo de proteção).

In order to properly balance this encounter, calculate the effective XP of the fight as if there were 2 NPCs with 1/2 the health of your boss. This is literally what it is, it's just that they share the same HP pool.

There are several ways to balance for a larger party...

The simplest is to use the balance rules in the DMG or DM's Basic Rules, and pick a suitable point total for your party.

If you want to adjust a single critter, adjust its hit points up; keep in mind that this should adjust its CR and thus its XP value. (Once you learn the monster rating rules in the DMG, you'll know how to adjust the XP value.)

Lair Actions and Legendary Actions are excellent tools for adding damage potential to a monster without changing its basic toughness. This, also adjusts CR. Again, the DMG will help you adjust the CR for them...

Another option, for group encounters, for every four monsters, add 1 per PC over 4. So if the encounter is 10, add 2, maybe 3.

More risky things to do -

• replace the weapons with bigger ones,
• Increase the AC
• increase the Attributes (and thus saves and possibly attacks)
• Give multiattack

Replacing weapons with bigger ones can change the CR. As usual, see the DMG for effects... remembering that +2 sides is +1 average damage, and +1dX is +(1+(X/2)).

Increasing AC makes the critter more survivable; it needs to be done in moderation. If you do it too often, it becomes annoying, and if done too far, it becomes totally disheartening. The occasional boss, however... sure, go for it. Give him suitable armor, and buff his CR.

Increasing attributes again has the potential to buff the CR, but isn't as likely to do so as increasing damage. Again, the DMG tells you the impacts...

And remember: Not all encounters should be "Balanced medium encounters" — some should be tougher, some weaker.

Scaling is not always necessary for things like this. You can also do things like adding tactics to the monsters, which can make even the most innocuous monsters into lethal killing teams. Tuckers Kobolds are a great example of this. Aqui is a link to the 1d4chan article on them.

That being said, it may be tempting to create the "super monster" that will give your players a rough night when it walks into their camp. I have fallen prey to this and it rarely works lol. Instead set up ambushes. Create squads with themes like goblin archers that have nets . This can cause havoc if even half the party gets webbed in place as the goblins pick off the mobile ones with arrows. Suddenly goblins become a interesting encounter again.

Or as a single monster encounter create a single level three goblin sorcerer giving it the invisibility and web spells as level 2 spells, and burning hands as a level one spell. The mage uses invisibility to hide in an advantageous position. Once the players move into the ambush area, it immediately casts web on the party, then quickens a burning hands to cast immediately. the party must save for half or take 3d6 fire damage, then as the burning hands ignites the webs the players all take an additional 2d4 fire damage they cannot avoid. Initiatives are then determined, and if the sorcerer manages to score a high roll, they can open up with another two possible burning hands (including quickening), dealing 3d6-6d6 more fire damage that can be reduced with good saves. This causes the party to take 9d6 from the burning hands and 2d4 from the webs that was unpreventable before they even get a chance to take an action, causing most of them to retreat (the goblin will likely also do so as it spent it's load) from a singly lowly level 3 goblin. (I know this is quite lethal, but it is an example to show what you can do with a single low end monster)

Without knowing your group composition exactly, I can tell that your issue stems from having 2-3 more players than is intended for the Challenge Rating of the encounters you provide

... especially if:

1. You are generous on short/long rests between encounters or
2. You build encounters based on an average group size of 4 players.

6 players is 150% of what is considered for balance purposes. Balancing a ruleset often comes down to both macro and micro (mathematical) averages. It's as much about balancing the ultimate 4 players group composition as it is about allowing for some malleability in case where you have more or less players in your group, or a sub-optimal group compisition (eg. no healer or all STR builds).

That should be taken into account.

It is not just about rules and game mechanics

You might not like it, but having more players automatically slows down the flow of the game. It is only natural that more people interacting equals less spotlight for every individual ...

...that also means having to scale encounters up. Like many good previous answers suggested, there are many ways to do this, especially if your concern is about not wanting to further slow down the pace of the game.

Don't shy away from giving your 'encounters' random boosts.

Enlarge them, give them some kind of damage resistance or lair actions, gives them more intelligence and/or tactical capabilities, more AC, better attack rolls or go crazy and give them a poison attack (but be careful when boosting raw damage output ... it's double edged). Mix those 'solutions' up to keep things interesting and keep your players on their toes.

Put them into a disadvantageous terrain. Force them to fight in bottlenecks, use difficult terrain to restrict their tactical movements possibilities, use traps to soften them up, etc.

Also ... know your group. Litterally all characters have weaknesses. This can be exploited, but shouldn't be abused. For example, learn to force your cleric to take DEX saving throws or your fighter to take WIS/CHA saving throws. Temporarily disabling even just one character can be the difference between an easy fight and a dangerous one.

Do not single out one lone player (like focusing the cleric, story of my life) but take turns at building encounters that will challenge 1 or more PCs specifically.

Fun thing I've seen, which won't make it too much harder but could allow for crowd control, give your boss a class. Immediate recommendation would be a level of sorcerer for a few spells that can cause crowd control but you can always do multiple levels of whatever class you want. I also recommend adding the health that would be added by a class.