Supposedly not enough storage to back up device--but using much less than capacity

I have pared down much of what was stored on my iPhone 13 (deleted most photos, since they accounted for most of the space), and marked most apps as Off for the backup, but I still get the "You don't have enough storage to back up this device" message. Phone says I have 5GB storage, with 3.6GB in use. Backup was attempted multiple times, connected to power and wifi, screen off, but no luck. Since I am using much less than the storage available, why won't the phone back up? Any recommendations will be appreciated.