I finally spent some time on learning about autotiles (with this explanation: https://github.com/godotengine/godot-docs/issues/3316).

I'm still not sure how to share the tileset so it's both ready-to-use to use and engine-agnostic, but setting up the whole thing shouldn't take more than 5 minutes (at least ing Godot) so I hope it'll be helpful to someone anyway.

The walls are bit different (thickness and such) than previous ones, so keep that one in mind.

In case you want any wall than the shorter ones (16x16) are dead-simple to use.