Question about assimilating hives into individuals

I recently had an opportunity to claim and conquer all the worlds of the shattered hive FE without my allies getting involved. The helpful idiots were busy with another war and were far away. I don’t like orbital bombardment, for a lot of reasons but mostly because it takes far longer than just dropping 50k worth of troops on the planet and moving my fleets. AI empires love parking fleets in a single system and making wars last forever. It also destroys resources that I want, pops. I ended up getting a little over 180k worth of hive world habitability pops that I began assimilating. This takes a while and the last time I conquered them I ended up only netting about 60k and I think it’s cuz I terraformed the hive worlds where the assimilations were happening but I don’t know, so this time I didn’t mod any pops and didn't terraform any worlds and netted 537 total pops, out of 180k. that’s not a very good deal at all. My question is, how do make sure I keep more of the pops? A second one is, can hive world pops live okay outside a hive world? a lot of the assimilated ones resettled themselves to other planets but they eventually evaporated too. I know I’m missing something, just trying to figure out what. The assimilation is from Bio ascension.