TRIGGER WARNING: HEAVY HOMEBREW! Campagne ending BBEG fight. I need help with mechanics and consistency.

If one of my players reads this please for the sake of our campaign stop reading.

Disclaimer: English is my 2nd language so dont expect poetry here

So our Campaing of 2 years is nearing the final Confrontation between good and evil and I want it to leave a lasting impression on my players. The BBEG is the leader of a plague cult named the "The Maggot King" and is a wizzard style NPC with a plague theme. The thing about him is that he is actually just a parasite that got control of an incredibly powerfull wizzard.
The fight is a classic good vs evil with a big bad ritual, a skybeam and all the other lovely clishes we love about fantasy.

So far I was thinking about 2-3 big phases:

Phase 1:
BBEG is busy with his ritual and untouchable. The PCs have to get through his minions and destroy 3 "what evers" to stop the ritual and make him vulnerable.

Phase 2:
He loses controll of the ritual, skybeam/chaotic magic everywhere and a fight against the wizzard till we trigger phase 3.

Phase 3:
Skybeam lifts big chunks out of the ground/Buildings and the rest of the fight the PCs have to navigate flying "Islands" that fly around that skybeam/vortex thing.
The BBEG completly loses controll of the ritual and mutates through chaotic magic into some kind of insect/poison-dragon-monster thing.
The rest of the fight is going to be getting the monster down and dont fall to your doom.

My problem is that I realy like the idea of the BBEG beeing a parasite that has to be exposed within the host to truely be damaged but the more I think about the dragon-mutant the less I like it because it feels inconsistent. Also I get the feeling its getting a bit much at this point.

Do you guys have ideas on how to build this final bossfight better but still keeping the basic themes?

Thanks for everyones response in advance.