iPad optimized photos selected but still no room for updates

I have "optimize photos" selected on my (16GB) iPad but still don't enough room for a needed software update. I'm paying for 2TB of iCloud storage including Photos and I've used 300GB mostly in pictures and video and to a lesser extent iPad and iPhone backups. The iPad is synced so I can't delete any pictures without deleting the original from iCloud correct? Should I take the iPad offline, erase all content and then selectively allow only certain items to be restored form a backup? Thanks!