I loaded this mod on an existing save. The planet Nauvis seized to exist. No messages, no errors. Just disappeared. All other planets are fine and working.

I think that's intentional. The mod mentions it replaces Nauvis in the Information tab (not in a very prominent place)

Yeah, Version 0.0.1 still has Nauvis in it if you want to load in an existing save. I'm currently working on the Solar System++ modpack and will remove all my planet mods doing the replaement themselves.

Any chance to add this mod as a standalone? Just the planet? I'd love a late game uberplanet of bugs to fight with endgame material but none of the dependent mods interest me.

Quality bugs and nest ? :D

Logic for removing planets has been removed as of 0.1.0 versions. I'll look into changes to Nuclear science pack dependency to make it optional if you'd like @Scarlet_Plague

I've had to lower the difficulty a bit, since players were traveling to Hexalith ill-equiped on the playtest (and failing to secure a landing site.. I'll look into ramping up difficulty (like make pollution from Uranium mining huge.. and have quality bugs/nests or power up the mobs post landing)

I'd very much like it so that I can add this planet to my save without it bricking my entire factory ahah.

As for additional difficulty, you could maybe look into adding Rampart behaviours into the biters of the planet? Making it like a separate class (Alpha Biters or something) and having them behave aggressively by default. Sending raiding parties to attack your base regardless if you pollute or not, preparing waves and sending assaults at regular intervals, that kind of stuff.