LEET, Kebes, Flurry, Raelin, and Griffin gtfo pls

Hey r/DMAcademy,

While it's a ways off, I want to plan my end of campaign boss fight. Here's a quick little bit of backstory;

The players are fighting in the galactic frontier against illithids and Mage Hand Press' response to them, the wroth (people-stealing boogeymen featured in their supplement for 5e, Dark Matter (highly recommend).

Modeling this campaign off Gurren Lagann, Gunbuster & Diebuster, Star Trek, Space Dandy, and other wacky/over the top sci-fi, I want an incredibly bombastic finale. "All the stars in the sky are my enemy" type deal.

1.) How do I give them this feeling without an immediate TPK?

2.) How do I do a three-way fight where the two bosses are fighting each other too?

3.) There's no real huge boss-fight monster with either the illithids or the wroth, so I was planning on homebrewing my own. Does anyone have any inspiration for some abilities I can give these guys?

I'm just about at the point in the campaign where the hero curve is going to start getting exponential, so I still have time to implement some pretty long-term ideas. I would say I've got about a year left irl before we're done with this campaign.

Thanks!