Devs haven't programmed in a final upgrade nor a final scene with Beatrix yet, said they'll get around to doing it sooner or later.
Come to think of it, the entire endgame feels like it was haphazardly slapped together despite the project being delayed multiple times. Umi Umi feels like a lot of empty space (with only two new enemies to fight) and Chaos Castle is nothing more than a Frankenstein's Monster of the game's previous areas and enemies. Hell, even the Chaos Witch (y'know, the final boss of the game) is a just mishmash of attacks from all the previous bosses, and that's for both forms of the fight.
Honestly, the devs should have cut Umi Umi all together and made Slime Citadel the final key location. That way, they would have at least had a bit more time to flesh out Chaos Castle and actually make it a proper area. It's incredibly frustrating to see the "release it now, finish it later" mentality of modern games spilling into the indie scene. There's nothing like dropping $15 on a game only to realize that the damn thing in incomplete because the devs couldn't wait a little bit longer before putting the thing on the market.