My most recent game was a Hive Mind. I think I assimilated some 75% of the galaxy, getting right to the endgame crisis before the most recent update dropped. I've kind of lost interest in the save, partially because I played with Gigastructural Engineering and that updated with Machine Age as I was playing, so the save file is a little borked. Looking forward to playing with the Cosmic Storms and the upcoming Grand Archive, though.

I think I crushed two Fanatic Purifiers and assimilated the Great Khan, so you're welcome, the rest of the galaxy.

There is somewhat of a case to be made for Materialist, considering Cosmogenesis and the Synaptic Lathe. But obviously not every materialist empire goes into that. To be a fanatic purifier, you need to be Fanatically Xenophobic and either Militarist or Spiritualist. I'd expect Xenophobic empires to not care (or care significantly less), but the other two ethics are a little more debatably nebulous as to how much they'd care.