Stop Photos Downloading Photos

So I’ve got iCloud Photos enabled on all my devices. My Mac mini is set to download all photos and has a storage usage of 48.2GB according to Finder.

My MBA has optimise photo storage on, but the photo library is currently at 30GB - so it seems I have almost 3/5 of my photos and media downloaded.

Is there a way to limit the storage capacity? I’m sure I heard somewhere that I could create a partition (or Container, as I’m using APFS) with the maximum size I want for Photos to use and then move the library over to that and it would never grow past that. Is this something that others have tried?

Now, I do have plenty of space on the drive at the minute, but I don’t want all my photos on the laptop - I’d have enabled download all if I wanted that!

The other option is to turn off Photos, which may well be what happens, but I do occasionally want to access photos on it - though accessing them via Synology Photos may be the solution if I can’t limit Apple.