F12 doesn't do anything, and due to both Shift and Tab being used in-game, I can't bring up the overlay during gameplay.

Not a huge deal, but I wanted to take a few screens of the lightning storms, and was a bit baffled as to how to do that.

I don't think Galaxy features are incorporated. Even when you get achievements you don't get a pop-up or warning.

As a bare bones method, you can always Win-PrtSc to take a screenshot... or if you have an NVIDIA card, use the NVIDIA overlay, you can take amazing screenshots as well as real time screen recording, etc.

With the overlay force enabled it leaves some big .tmp files in the screenshots folder: maybe they will sort it out in the future, since it has been a common issue with UE titles recently. With the overlay force enabled achievements popups should appear, try enabling it by following these instructions https://docs.gog.com/gc-overlay/#testing-overlay-locally

If you want to know more visit my forum topic here

I cannot enable it; it is unavailable for this game.

Nvidia overlay makes the game performance sink for some reason.

I use OBS for taking screenshots in game that don't provide their own means to do that.
It may look a bit complicated, if you didn't use OBS for anything before, but it works and there is no overhead (we won't be recording, just taking screenshots now and then).
First you set it as if you would want to record the game (of course you won't be recording a footage, but you need to set what program/window/desktop should the OBS "record", and set things like output folder for where the recordings/screenshots go, etc.).
Then go to "Settings" -> "Hotkeys", and make sure to set the "Screenshot Output" hotkey to something - then use that key shortcut to capture a screenshot while playing the game (don't make OBS start recording or anything like that, just let it sit in background passively, and just use the assigned key shortcut to make it take screenshots while you are playing the game).

As I said on my previous reply, you can enable it globally for every game by following these instrucions

https://docs.gog.com/gc-overlay/#testing-overlay-locally

Ah sorry I misread your reply. I'll give that a try, thank you