Check out Simple Photo Gallery. It generates static sites (photo galleries, really) that look both modern and pretty (and it's themeable) but requires nothing as a back end. An example of what one of the galleries looks like in the wild: https://drwho.virtadpt.net/images/museum-ramses_the_great/public/
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Maybe PhotoSync? I use it to upload to photo prism but it has s3 compatibility I haven’t used but it’s in the settings. The app is great.
That's the sort of thing, but PhotoSync is proprietary. Still, it's the first suggestion that actually fits my needs and doesn't require any self-hosted software, so thanks!
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.