Made another quick release of v17.3 to fix this issue! Not granting full access to the photo library should now work as well, although it will not work quite as well.

The main noticeable limitation is that if Toot! does not have photo library access, then photos will not be pre-selected if you go to change your selection of attached photos. You have to delete them from the posting screen and then re-select them to make changes. This seems to be an Apple limitation that can't be worked around.