hi leonie.
thanks for your response and three points...
1/
i'm looking on my MacBook pro: system prefs. > <my name> apple account > icloud > manage > manage storage > Family usage.
2/
your hunch is right. i back up to a time machine drive. SO. i'm currently restoring the photos library from a snapshot i have from the day before i started deleting large videos and slomo files. this is because i'd only download modified not originals (stupidly). before starting this restore just now, i exported (originals) of all photos media since the date of that photos library that i am restoring, as i understand it's all or nothing and don't want to lose the new media. so if that restores ok, (happening now) i will then re-add the interim media mentioned. then i will re-commence offloading media (originals this time). NB - i'm slightly bemused that the snapshot i'm restoring is 1/10th the size of the current photos library. i opted for 'keep both' rather than replacing the current larger one. so nothing will be lost, in theory. thanks for posting out the time machine backup and all those snapshots. i get that when i'm fully happy with all this i could delete my time machine back up and start a new one. and maybe that will finally clear down my nearly 1TB storage.
FWIW i paid a one off fee for the desktop app PHOTO SORT which enables creation of an album of your largest files in Photos allowing you to set a total storage amount as a target. eg show me 300GB worth of my largest files. then these can be exported, sorted by size, the whole album could be deleted from Photos, emptying the recently deleted album. then any wanted in iCloud re-added from external drive back to Photos library. the app is here in the apple app store: https://apps.apple.com/gb/app/photosort-size-quality-sort/id6739038077?mt=12
in answer to your final question, i have shared albums - a fair few. but not a shared library. we use family sharing... each of the 5 of us has their own separate photos library tho.
i hope this is all clear and thanks for your input. i hope this helps others, and will follow up with final observations. BUT APPLE - please gives us control over our own data - let us sort by size, and suggest more deletion. let's not encourage filling up more and more heinous data centres with more data than we need to, whilst spending more and more money on cloud storage.
if this all doesn't work i'll try yours and mark's suggestions of checking the media on desktop iCloud.com, and my phone etc etc.