You might want to look into Disco Elysium. It's not exactly scifi or post apocalyptic in a traditional sense but is certainly adjacent to it if not in a subset of it. It plays similarly but is more akin to the gameplay of Planescape: Torment.

It's at least worth checking out some reviews, descriptions, screenshots, trailers, etc. It might not be your thing but as a Fallout 1 and 2 fan it was right up my alley and is easily one of my favorite games of all time!

Not exactly the same but check out Kenshi. It's also more like Wasteland than it is like FO but i can't recommend it enough.

Post apocalyptic weird low-tech scifi sandbox.

Important to note, its completely open ended. No story. But quite a lot of stuff to do, factions to keep tabs on, settlements to build and manage, people to recruit.. lots of moddability.

You'll have to find your own purpose out there.

Oh and it's crippling difficulty can be daunting.