If you don't mind older games you could also try:
- Starflight (never played but it's considered a classic, altho it's not a pure rpg but more of a genre mix instead). There's the original Starflight 1 + 2 on GoG or this remake thing
here which also supposed to be pretty good.
- The Buck Rogers games in the Gold Box engine, there's also a version for Sega Mega Drive / Genesis which plays a bit different
- Albion has some Sci-Fi elements in it and is generally a really good game
- Whales Voyage 1 & 2 which include space travel, trading, and sci-fi dungeons
- Worlds of Ultima: Martian Dreams which is obviously Ultima VI on Mars (in the 1890s)