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.
The ball is in Apple's court.
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.
Not really! The advent of APFS volumes makes Prtitioning easily avoidable. 😜
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.
If you absolutely have to have a clone how about a 150GB partition for the clone and the remainder of the drive for Time Machine?