Optimise Storage With iCloud Not Working Properly

Is there a way to trigger offloading photos to iCloud? Or at least where can I read about how exactly this process works?

I recently switched on "Optimise Storage" and bought a 2TB iCloud, because I kept running out of storage on my 128gb iPhone 12 mini. All my apps, data and system files take up ~30gb, and the rest I keep filling up with new photos and videos. When I first bought additional iCloud storage, most of my photos and videos were optimised and I was happy. But then I shot a lot of new content and my storage was again full. I've been trying to make iCloud do its work again, but to no avail. So I just had my phone full for a few days and I couldn't do anything, had to manually delete apps just to be able to take a picture. Then in a few days out of the blue it optimised storage again. Then in a few days I filled it up again and now it's not optimising, and it's been 3 or 4 days. I only used up 300gb in iCloud so that can't be the issue. Obviously tried on/off, leaving overnight plugged in. Running latest iOS 14.8, updated as soon as it came out.

This is becoming a very annoying part of my life, I need to sort this out ASAP or change the phone, so I'd appreciate any help. I need to know if I can make storage optimisation work as promised or if I'm doing anything wrong.

I know there's 3rd party apps for this, I know I can do stuff manually, but honestly as a long time iPhone user I got used to the fact that everything just works, not to mention I paid for the iCloud extension and I think I can expect it to do what I want.

Thank you guys!