The Steam Next Fest is live and will run until October 8 and allows access to 720 free demos, 122 of which are sci-fi. Additionally there are 95 total and 21 sci-fi demos for unreleased titles.

You can find sci-fi is under "Themes and Moods" in the filter.

PC Gamer have published their 12 favourite, including 2 sci-fi titles, Exo One and Starship Troopers: Terran Command.

The Planet Crafter looks to be Subnautica inspired, both in gameplay and appearance. Though above water and much larger in scope with planet terraforming.

References

PC Gamer (October 1, 2021). With hundreds of demos at Steam Next Fest, here are 12 of our favorites. 

Steam. Exo One. (viewed October 2, 2021)

Steam. Starship Troopers: Terran Command. (viewed October 2, 2021)

Steam. The Planet Crafter. (viewed October 2, 2021)