Google Photos already has a public API. But you cannot consume it if you are building a competing service[1]. Also, the APIs will not serve you files in their original quality.
If they wanted to honor the intent behind GDPR's portability mandate, it would have been trivial for them to enable seamless migrations.
Now what's ironic is how Google is a "partner" in the Data Transfer Initiative[2].
[1]: https://developers.google.com/photos/library/guides/acceptab...
[2]: https://dtinit.org
OneDrive ceased to be a good backup alternative when they stripped GPS from all my photos 1-2 years ago. Unlike Google, there's no way to recover original files. They simply destroyed them.
This is scary. Isn't OneDrive supposed to keep files bit-identical? Or were you using their iOS/Android apps to back up photos, and some bug caused the photos to be backed up without their metadata?
Their mobile app is now stripping GPS on upload while files uploaded from desktop are unaffected.
However the files I already had uploaded going back to 2017 were stripped. No idea if this was done server side or by the mobile client but the end effect is the same.
Unlike Google, Microsoft doesn't bother documenting this policy.
But with takeout the problem is that all dates and metadata gets messed up. I had to learn this the hard way. There is some tools that somehow correct the dates. But it's not perfect and I wouldn't really want my photos to be locked up like that. What if google decides to discontinue Google Takeout? Or just give you lower quality pictures after some point? You really are not safe.
Since that experience I moved to onedrive temporarily because it's a bit safer solution for now until all of these new services get stable (immich, ente, etc). Onedrive let's you access you photos like normal Onedrive files so that's really good. I know microsoft will have my data now and sync is also not perfect. So it's definitely not perfect. But I had to move to something else before the better solutions emerge.