Not enough space for Photos Library on Mac

Hi. I intend to limit the space taken by my Photos Library on my Mac. So, I created a volume limited to 15 Gb, according to some suggestions I found online. My iCloud Photos Library is about 75 Gb, but I am using and had used the "optimize Mac Storage" option. I want to have my full library at a lower quality, so downloading photos as I need them is no big deal.

Before creating that volume, even with "optimize Mac Storage" on, the Library took up 50 Gb. For a 128 Gb MacBook Pro, it's way too much.

When I had less space in my Mac, the Photos Library local file was about 7 Gb in size, so I thought 15 Gb would be a reasonable space for the volume. But the synchronisation is paused because of lack of local storage. At this moment, the (incomplete) library is 1.84 Gb.

I've found a workaround that suggested unticking iCloud Photos in Photos' Preferences, and then ticking it again, but it didn't solved my issue.

Any ideas?