I hadn't have much luck at finding game on linux similar to Sins of Solar Empire: Rebellion (works well with proton). Then digging deeper i've found in my ubuntu repo:
- Endless Sky (Single player. Available on Steam as well. Open Source. Free)
- FreeOrion (more close to SoSE:R, inspired by Master Of Orion, LAN and Multiplayer, Open Source, Free, more complex than Endless Sky)

New Plasma 5.16 rocks!
BTW, new episode of Destination Linux is out, time to get a snack and turn on that Red... (that won't sound right as a joke...ok) YouTube.

LinuxRocks.Online

Linux Geeks doing what Linux Geeks do..