TL;DR: I've added achievements, you can turn them on in the options but I'd advise playing through once without them first.
Well, it took much more work than I expected, but I've added achievements to the game now.
What should have been a relatively simple job, (it's only a few lines of code for each achievement), turned into a multi-day process involving trawling through dozens of old forum posts. The advice varied from "rebuild the entire engine manually", to "tick one box", to "remake the entire game in C++" and nothing worked.
In the end, it turned out that UE4.26.2 broke Steamworks support.
Hence why this is such a relatively large update, I did have to add some C++ to the game to get Steam to work, but primarily the change is that I've moved to 4.27.2, which is also the final version of UE4.
As for the achievements, these can be turned on in the Options menu, and they're all pretty easy to get in a normal playthrough. I would advise playing the game without them the first time through, the game is short and built around its atmosphere, so the steam notifications kinda detract from that. After that it's a simple enough job to turn them on in the options and play through again.
Also, this update wasn't actually supposed to go live till today, but I accidentally pushed it yesterday. Whoops.
Finally, I just wanted to say thanks for supporting me so far. I'm about half-way to my goal with the game at this point, and honestly I didn't expect to even get there. Thanks for taking the chance on my game, and thanks even more for not refunding it after you beat it.