How to make a custom entity the subtype of another?

Hey, I'm trying to make multiple variants of a somewhat intricate mob, one that uses all kinds of synced data variables and such, as well as model layers that show depending on the values of these variables. This also includes many, MANY alternate textures that show under the right conditions just for the first base variant. Unfortunately, if I simply duplicate the mob, all of the procedures that do things like check the synced data, or change the values, simply do not function on the new mob, because the code builder is very specific, and has to detect synced data by mob type.

So, I thought, maybe it would be possible to simply make the custom variants subtypes of the main one, that way when it attempts to read the synced data of the variant, the variant will still count as a type of the main one that the code refers to, thus making the code still work.

However, I cannot find a way to make a new custom entity a subtype of another custom entity. This sucks, because oh my god, I do not want to have to rewrite several very, VERY long procedures. Any response is much appreciated.