It says it has achievements on the store page, but I would like to know from someone who owns the game on GOG. Does it have Galaxy achievements? This looks like one of those games that are not treated equal to a Steam version. I see there is no cloud saving either? What about mods? This all makes it harder to want to buy it on GOG :(

You're welcome. :)

I'm happy with buying it from GOG.
I haven't played in a while but I used to use a ton of mods. The paradox link is indeed the best place to get mods, especially if you want to save them offline to your computer.
It's also possible with a paradox account to download them within game.

Another place that used to have frequently new mods but sadly isn't used by mod devs anymore is Nexusmods: https://www.nexusmods.com/survivingmars?tab=updated
It works great with Vortex to mange mods, I swapped to manually downloading the mods from the Paradox website and manually installing them with Vortex.
A lot of options if mods is something you're into. :)