If you feel the need for a bootable backup, the Recovery Drive will work to make repairs or even reinstall if necessary, which is what Apple intends. (Big Sur at this point cannot be cloned unless and until Apple creates a utility to do that. SuperDuper and Carbon Copy Cloner can only clone the data volume.)
I'm having a LOT of trouble trying to believe that Apple is going to leave us with no way to create a bootable clone. I'm having a LOT of trouble believing that they'd even think of it.
Their mandating an Internet connection for upgrading was bad enough, but at least it only left us potentially unable to upgrade, which is an option rather than a necessity.
How do they expect us to recover when we NEED to recover and we haven't got Internet accessibility?
Whatever you decide, I do NOT recommend partitioning. (Okay, I NEVER recommend partitioning unless it is unavoidable.)
You've softened that stance since the advent of APFS volumes.
In anticipation of CCC and SuperDuper! regaining full functionality, and since Rita's got a 1 TB external to potentially use for a Time Machine backup of a 120 GB internal with less than 40 GB of system and data combined, I'll stick with my suggestion that she split the drive in half and reserve 500 GB for Time Machine and the rest (I suggest 5 x 100 GB.) for backup.
