Alien Dimensions is open and looking for your hard science fiction stories. I like how the submissions guidelines page has a tl;dr section at the top:
3,500-4,500 words only. “Set it in space, in the far future, and include some friendly non-humanoid aliens helping to solve a pseudo-scientific problem. Written in the third person.” US$10 flat rate.
That doesn’t leave me much to add, although there is some more detail:
At least one great idea
Preferably written in the third person (or if you prefer first person writing, do a lot of head jumping, and include lots of dialogue)
Set in the future, at least 100 years hence. Happier with 1000 years hence. Let’s avoid 20th/21st century baggage and just tell an original story
Some kind of non-humanoid aliens present, usually friendly
It doesn’t pay a lot, but if you have stories of this flavour it might be worth checking out.
Literarium listing here: https://literarium.net/market/alien-dimensions/alien-dimensions