Spent the evening with a good beer, and deep dive into Garage (https://garagehq.deuxfleurs.fr/) , a small, lightweight S3-storage daemon that does the job.
@skoranda If you'll allow me to pitch in, I've been using Garage for almost 3 years now and it's been pretty nice. Managing it can be a hassle at first, since it's CLI only (and some of the commands can be a bit "annoying" to understand) but once you get the hang of it, it's pretty simple.
Updates are very simple (just create a backup, replace the single self-contained binary, and off you go), it has been very stable and fairly performant (for as performant as my server's storage is).
I haven't used any clustering yet but it doesn't sound difficult to set up either.
The community on Matrix is also very friendly if you need any help.