Unable to turn off iCloud photo sync due to storage issues on iPhone

my mother and i use the same icloud account and recently our photo libraries have been shared too. this happened a few years ago and i managed to turn it off but this time i cant seem to turn it off again.

i tried turning off the icloud photos sync but was told that i can’t download the originals because i would need an additional 14.16GB to do that. on my mothers phone it also says she needs additional space to turn off the syncing.

my phone is 128GB and i’ve only used 62.74GB of that (1.56GB is photos). my icloud storage uses 26.2GB (21.71GB of this is photos) of 50GB so i’m just not understanding how to fix this? i’ve already deleted the photos that i can to make up space and i’ve deleted apps and some backups but i do not have anything else i can do to get more storage for this.

[Re-Titled by Moderator]