I’m not 100% sure if this meets any of your requirements but I always enjoyed homeworld can’t remember if I’ve ever run it on Linux since switching tho.
As to the other requirements, I don’t think I found it that easy to get into as it took a while to get into the whole 3D map thing, and the graphics may have aged
I’ve used them and can find a lot of stuff that isn’t available on the more open seas, but maintaining ratio is a big issue, not sure if it’s my setup or the tracker itself, so I can’t download as much as I want to.