I'm only just beginning my exploration of solo TTRPGs, but Alone Among the Derelicts was a great game to start with. The mechanics are simple and easy to understand, and I didn't even need a physical dice or deck of cards to get started, just online versions. 

The fiction prompts are simple but evocative and create great fodder to generate very interesting stories. I can get overwhelmed by pre-established lore in some games, but Alone Among the Derelicts creates just enough of a setting and stakes to get you invested without bogging down or constraining you with details.

The world it sets up is easy to understand, and whatever protagonist you create with your writing, the game quickly makes you care about their salvage mission and think deeply about their headspace as they search through the derelicts.

I've only played one session so far, but you could probably play this for a long time and not get bored. It also is open-ended enough in its setting that playthroughs of Along Among the Derelicts could be incorporated into other solo or multiplayer sci-fi TTRPGs.

I definitely recommend people check this out!