It's great!
We have had file systems for decades. They work well. They sync trivially.
But Photometor.app (owned by Apple) can...
So that's a little annoying... I wish I had more visibility on photos not showing up in Photos.app, and what it is that stops it showing them
Only need to go to this page to do the request https://privacy.apple.com/
Guess I should've searched harder!
Going forward, you’d want to set up some other way to sync photos you take from your phone to your other devices. I can personally recommend Synology Photos for simplicity[1], or Immich[2] for an open-source (and in my opinion, slightly better) alternative you can run on any hardware, if you’d like to set up an always-on NAS. These are “Apple Photos” or “Google Photos” equivalents that you host yourself.
Alternatively, something like Syncthing[3] is a dead-simple way to sync your photos to various other devices as and when they are online, if you’d prefer to manage your photos in an ordinary file manager.
I’d be remiss not to mention that, for any solution where you move off the cloud to a central storage location of your own, you really must make backups to keep your photos safe. The 3-2-1 rule is a standard recommendation.