My 2cents. I would set the free space to something other than 0. For instance I know my Typical backup size for my Dockers are around 36GB so I set my Minimum space to 50GB so I know that when a drive gets that low it'll automatically pick another drive.
I also normally target a specific drive for all my backups. I use disk1 for all my Restore/Backup stuff, but that's just me and in hindsight might be a bad idea since if that drive dies no other backups are anywhere on my array. Lol, I might have to rethink that, but It'll do for now since I do have offline backups too.
I know what I typed up above contradicts each other since I pick a drive, but I use the Minimum Space on all my shares to insure they don't get full.