ICloud – After disabling iCloud Photo Library, cannot re-enable due to insufficient iCloud Storage

So after some time (learning python on how to monitor and plot library size and disk usage) this is what I have observed regarding my first question. As written by @will-mallard and @bmike the optimizer targes for percentage free disk space. As shown below: the size of the picture library increases and decreases together with my disk usage. During those three days I did not use my mac book air for work. I only added pictures two times.

Over a longer time scale, one can see the reduction of picture library disc space over time while I was working on my macbook.

My followup question is now: What triggers the optimization? At first I thought it is some kind of free space threshold. But as it is easily seen in the last plot, something at the 24th May triggered a reduction of library space. I am not sure what that was.

I have an iCloud library of about 25000 photos and videos, running on multiple iOS devices in "optimized" mode. Each device had to go through the complete backup process of uploading every image and video. Google Photos' iOS app will upload images first, then videos.

Which creates some problems on the iOS devices: With the "optimized" setting iOS only keeps the most recent or recently viewed assets in full resolution. Since google photos requests 100% of the full-size assets the iOS device, in my scenario, inevitably runs low on space for as long as google's upload process takes. Very. Annoying.

But fortunately I don't get any duplicates in google photos. I wish google would figure out a way to intelligently avoid having to upload what's already there, as this process, with a library of +150GB takes weeks.