Final boss really hit you with the wait, holy shit what do I do now? I do agree there's some rework to be done to make it flow better with the game. I've found that the defensive weapons like energy balls, fields, and even the girls shooting cum don't really play in any sort of manner. Kinda random or roughly positionally based. I don't believe the boss needs to be difficult, but rather keep you running back and forth since the map is so big. Plus the boss has double the "tanking" ability.
It would be nice to see those orb things go away for this boss, and the succubus' who casts them. I feel other defenses or enemies would tie better with the boss. The two girls on the side with the parasite dicks should be constantly releasing cum which forces you to run but, you could sabotage them and shut them down for some time if you have sex with them (taking a parasite charge and health damage so it's a trade off but fun interactable).
I also see how you believe the tube girls don't serve a purpose. I think if this mechanic is used you should be able to grapple them (not shoot) and stop them somehow. If you don't and they reach the boss and get absorbed then her health should be greatly healed by say 25% which makes you have to deal with them.
For the enemy spawners, I'm not sure how the succubus ladies play into the boss or even the story and they seemingly would be better replaced by other parasite controlled enemies. I would think any would do like the various parasite girls, or one that drops parasite eggs (kinda like the angel but not her). These would rotate and spawn on random chance making you change your play style slightly through the battle.
The boss should also have a breast phase attack. I'm thinking it replaces the laser beam that goes back and forth and she could either milk her self releasing a milk wave that forces you to the sides. Maybe, she gets a grapple move and forces you to sex her breast and if you cum you heal the boss similar to the tube girls.
My 2 cents on the final boss. But also enjoyed the game a lot as well.