The primarchs were genetically engineered and space marines are supposedly derived at least in part from their DNA. I don't think the imperium counts emperor-ordained modification as a mutation.
Or it's just overlooked because of their usefulness, like they do with the mechanicus and their worship of the omnissiah.
It got 14 new maps last year and hit a new concurrent player count record. It's not even remotely dead.
Adrian Tchaikovsky's Final Architecture trilogy ticks a lot of the same boxes as the Expanse. A blue collar crew of misfits from different factions gets caught in the center of a galaxy-spanning fight for survival.
The picture is an album cover from the band Brand New. There's a song on that album called Jesus Christ.
You can do a custom distribution. It does look like Humble takes a 30% cut minimum now though.
Check out Blindsight by Peter Watts.