Syncthing.
Just sync your DCIM folders.
You can enable “Ignore Delete” on each phone if you want to ensure photos aren’t deleted on the source phone if the destination deletes it (or not).
I sync stuff to a PC at home, so phones don’t have to be on at the same time - they’ll always sync to the home PC.
That’s a Frickin’ Land Yacht!
I’ve driven a few beasts like these… It’s something else