Ok im 99.9% sure this won't be implemented due to the time it may take but why not add wither storms with diffrent command blocks it can be like this
Impulse - Easy
Chain - Normal
Repeat - Hard Mode
Ok that's all for right now

It's probably not that hard for you at least it's just make new entities and retexture the command blocks and I'm 50% sure phases 4-7 don't need to be texture modified

Wait hold on this idea is still alive???

actually considering my command block knowledge, it would make more sense for chain to be easy, impulse normal, and repeating hard. this is because chain command blocks on their own do nothing and require a command block behind them to run, but when the first command block is run the chain command blocks after it run as well (provided they are powered or set to 'always active').
also perhaps chain wither storms cant go past stage 3, but could join with another WS to boost its consumed entity amount and grant it special abilities. also boss battle adaptions for that.

Maybe if this is added, there could be different color config files for each type of wither storm

I thought there wouldn’t be chain and repeating wither storms.