Apple One storage is full. I want to erase 10,000 photos synched to an iPhone from Aperture on a 2014 27" iMac. Can you help me restart the iMac or clear Photo's 'From a Mac' albums?

Four family members are sharing 250GB of iCloud storage and I've been getting almost full messages for some weeks.

One of the iPhones we use has all these photos synched from the old iMac. I can't see a way to erase them on the iPhone and last night after hours of trying I managed to get the iMac to start. I plugged in a copy of the Aperture library on an external HD and used iTunes to switch off the sync for photos from the Aperture library.

It looked good. iTunes started creating a back up of the entire iPhone to the 2014 iMac and I saw it start to sync minus the photos. It was 4AM and I left it to finish the job and went to bed. In the morning before I'd had a chance to check the results, my daughter unplugged her iPhone and went to work. The iMac's screen was black and unresponsive and remained that way until I unplugged it. When I next had a chance to check the Photos app on my daughter's iPhone I found that the 10,000 photo album was still there.

I've tried unplugging the iMac and pushing various Apple-recommended keyboard combinations, pauses and despite endless Power plug pushing the 2014 iMac just won't restart to give me another chance to sync the iPhone.

Before any of this was attempted I took a complete back up of the iPhone to a 2021 M1 iMac.

I'd appreciate any advice regarding possible ways to restart the old iMac or somehow erase the bloated 'From a Mac' Photos folder(s).

Thank you.