1: I have been using subfolder of /mnt for different things when self-hosting. Different external drives go in different subfolders of /mnt. Example: Media drives are mounted at /mnt/media, data drives at /mnt/data etc.
2: I'm lazy. Mine are located in my server users home folder. I then use scripts to sync between them between desktop and server.
3: Just make sure than your server user, the docker user and root user can all read and maybe write to them.
Nice. Any plans on mobile clients?
This would be great for my spouse, but she don't really use desktop/browser apps. A mobile app could also integrate with the existing reminders/notifikations of the OS it's on.