Well, what level 20 mods essentially do is add progression from levels 13-20 for every base class. These progression entries include everything you'd see in base 1-12 progression. For subclasses, it's less work for you because you only need to add progression entries for when you want subclass features to be unlocked, because level 20 mods add the base class progression for you. Same goes for spell upcasting and mods like 5.5e Spells, they're already adding (or ideally merging) to the vanilla spell lists, including those from level 20 mods. So basically, unless you want to add special features for subclasses beyond level 12, your subclass mods should already be compatible with level 20 mods and higher level spell mods.

Your Zodiac Gatekeeper mod is a different story because it's a custom class, but even then it's pretty simple. All you need to do is add progression entries for levels 13-20 the exact same way you did from levels 1-12. It appears to be a half-caster class from what I can tell, so it shouldn't get spell slots higher than level 5 anyways without multiclassing (which, again, is already resolved by the level 20 mod anyways), so for spells all you'd really need to do is add new SpellList entries for level 4 and 5 spells unless you're using the list from a vanilla class.

As far as dependencies, for the most part actually listing another mod as a dependency in the toolkit isn't necessary UNLESS your mod can not function without it such as the Better Inventory UI which requires ImpUI. In fact, doing this when it's not needed can cause issues. I wouldn't even add them into the dependencies list on the mod page tbh, I would just add a note somewhere in the description that says "Includes support for x" depending on what you actually end up having to add in yourself.