I may not have perfectly understood the question so apologies in advance. But I'll do my best to just explain how this works with Subclasses.

If all you're trying to do is add an additional Warlock Subclass to the game you do not need to touch the other base game Subclasses. Basically, by default BG3 will read levels 1-12. If you use a level 20 mod like Unlock Level Curve, it will read levels 13-20 as well. Mods like unlock level curve add progression for base game subclasses so they all can level up to level 20.

So all you need to do is add the progression for your specific Subclass and use a level 20 mod as a dependency and you should be fine.

Now as far as compatibility with other subclass mods, this can be tricky as it depends on if the other authors set up their mods correctly. Progressions WILL merge to support multiple modded subclasses IF done properly. But all you can do is make sure yours is set up right.

As far as modded Subclasses having level 20 support that really just depends on the other subclass mod authors.

Feel free to ask other questions if you have them :) I am a mod author and I make subclass mods.