The last backup could not be completed

I have an iPhone X that for 6 months now has eventually gotten stuck where it can’t backup anymore, with the message “The last backup could not be completed”

When this happens, Photos sync stops working too, but most of the rest of the phone is fine, including Messages.

We can temporarily fix it for a day or two by signing out of iCloud and signing back in again, but it always fails again eventually. That is to say we can get one or more successful backups, which we can then use to restore, but subsequently with no changes to the phone, backups will fail.

We’ve tried to disable most things related to backup, and there are barely any apps on the phone, we’ve deleted most of them.

We’ve tried deleting the backup, backing up and restoring to iTunes instead (where we discovered the backup on the Mac was unusable because it was encrypted with some password none of us knew, even though the setting was disabled in iTunes). We have factory reset and even full restored from iTunes.

We’ve called Apple Support multiple times, patiently letting them walk us through all the same network setting rigamarole, to no effect. Someone said they’d escalate, but we’ve heard nothing from them since.

At this point, we’re about ready to give up. Does anyone have any ideas to try, or is this phone a write off? And does anyone know if we’ll have to write off the account too? Could it be broken in some way? We have an iPad hooked up to the same account and it seems to be ok for now. This is the first time in a decade I’ve been this disappointed in an Apple product, and not going to lie, it really stings.