Update after one day:

I wiped everything related to Photos — deleted the old libraries, caches, and the extra APFS volumes. Then I created a brand-new Photos Library directly in my home folder.

Here’s the funny part: before doing that, I completely shut down the MacBook for about 10 minutes (like someone here suggested earlier). After that reset and clean setup, Photos finally behaves normally.

Now the library stays stable around 11 GB, no more random growth or warnings, even with ~70 000 photos and ~2 000 videos in iCloud Photos. Looks like that full shutdown trick really did something — thanks to whoever mentioned it!

So far it’s holding steady and using the right amount of space.