I make a weekly batch of homemade ice cream using:
3/4 cup sugar
1 cup whole milk
1 pint heavy cream
salt / vanilla
I want to try including some nutritionally dense ingredients like butter, egg yolks, peanut butter, avocado, etc.
Is there an easy way to know how much of a given ingredient to add? I still want the ice cream to be enjoyable
Are there any other ways to increase the calories per batch?
I've tried using half and half instead of milk, but the texture is too "whipped."
Those ingredients you mentioned, you wanted to add, are all possible, i guess. Fat tends to make the ice cream creamier, even at low temperatures. There are many recipes out there involving gema de ovo, though, because it has other properties as well. It serves as an emulsificante, which binds water to fat. This prevents separation. There are other emulsifier, too, but in my opinion egg yolk gives the best results.
As we are not a recipe sharing site and it is definitely a matter of opinion how much of what to add (my dad adds about ten egg yolks for each liter of ice cream, but that's too much for my taste in most cases), I leave you to find yourself the best recipe on the internet. :)
Edit: In order to stay safe, it is recommended to heat the egg yolks to at least 70°C, but do not cook it though! It shouldn't solidify! I always mix it with sugar and heavy cream first, to prevent premature clotting.
I've tried using half and half instead of milk, but the texture is too "whipped."
That gives you a starting point, although you'll have to look at labels and weigh things to get it precise. If we assume that your heavy cream is 36% fat by weight, the milk is 4% fat by weight, and that instead of measuring by volume you had measured by weight in a 2:1 ratio, you're happy with (2 * 36 + 1 * 4) / 3 = 25.3 % fat by weight, discounting the sugar; and you're unhappy with (2.5 * 36 + 0.5 * 4) / 3 = 30.7 %.
So if you want to add avocado, look up the fat content (about 15%), and work out where that falls on the scale between milk and cream. Avocado's a bit under the midway point, so you'd add e.g. 100g of avocado and remove 65g of milk and 35g of cream.
Alternatively, with things like avocado and peanut butter, chop them, freeze them, and add the frozen chunks into the ice-cream machine five minutes before it finishes.
Ice cream with eggs is sometimes called "French style ice cream" or "classic (old fashioned) ice cream". There are recipes how to make them, usually it involve creating "custard" with eggs yolks and sugar (called kogel-mogel, Zuckerei, or Goggelmoggel).
I've seen some recipes for ice creams with avocados and almond milk in the "health" part of instagram and pinterest but you should be aware that they sometimes just look good and recipes they give cannot give showed results.