Sorry I missed the part about you not wanting to self-host, the suggestions below are probably not helpful then.
I don’t know anything that comes free but is also not self-hosted. :(
Original Post
I was using librephotos for a while and I thought that was compatible with S3, but according to their photo app comparison guide it’s “unknown”. That comparison is a good guide to get ideas about other apps to check out though.
Another one I was using was the nextcloud addon Memories which just uses any storage you have in Nextcloud, which I think can use S3, but as I haven’t used it myself I’m not really sure.
Edit
Sorry I missed the part about you not wanting to self-host, the suggestions below are probably not helpful then.
I don’t know anything that comes free but is also not self-hosted. :(
Original Post
I was using librephotos for a while and I thought that was compatible with S3, but according to their photo app comparison guide it’s “unknown”. That comparison is a good guide to get ideas about other apps to check out though.
Another one I was using was the nextcloud addon Memories which just uses any storage you have in Nextcloud, which I think can use S3, but as I haven’t used it myself I’m not really sure.