Doing an atomic-swap sync of a folder (which is what a photo library is) can result in momentary doubling of size-on-disk. For something as big as a photo library that may not be acceptable/possible.
The solution (differential syncs with atomic update and with support for the version being diffed against changing during the sync) are quite tricky to get working reliability, and they get less reliable the larger the atomic unit of data is.
All that is also assuming that there's a way for the sync program to interpret what units (files or folders) the user wants to be updated slowly/expensively/atomically, which is itself very hard to solve without adding friction or foot guns.
APFS has copy-on-write semantics. I regularly duplicate my entire photo library folder. Just duplicate a large folder in Finder and you'll notice (a) it's instantaneous (b) it doesn't cause the disk usage to increase appreciably.
The solution (differential syncs with atomic update and with support for the version being diffed against changing during the sync) are quite tricky to get working reliability, and they get less reliable the larger the atomic unit of data is.
All that is also assuming that there's a way for the sync program to interpret what units (files or folders) the user wants to be updated slowly/expensively/atomically, which is itself very hard to solve without adding friction or foot guns.