Coming back here with an update: the problem has resolved but I lost ALL of my pictures in my save. This was definitely tied to photos and/or limits on storage space.
I tried deleting several saves as you suggested @EA_Solaire which didn’t solve the problem but seemed to get autosave working again… Then there was the disappearing milestones bug so I avoided playing my main save until it was fixed and instead played a different, much smaller save file. This one didn’t have any issues saving and was much faster overall.
When the bug was fixed I returned to my legacy save and found that every picture was black 😭, but it is saving properly again. I suppose I could probably load an old backup and retrieve a few of the pictures by saving them to my library, I might try this at some point.
It’s a huge bummer that I can’t fully use the photography feature anymore, especially since I have PLENTY of storage space on my console… I wish that this could be fixed, and that there could be some sort of warning before causing issues and deleting all the photos.