From experience, Catalina is even picker about the size of the Time Machine drive and I have no idea what algorithm is used to determine the minimum drive size.
It appears to me that Apple has made decisions that don't allow us to use Time Machine according to the original Mac philosophy - Users should have a computer that let's them do things the way they wish.
I have absolutely no requirement to use Time Machine as a long range history. I just want to use a drive that is more than large enough to contain more recent files and, should the drive run out of space, Time Machine can boot the oldest files off the bus.
However, I found a while ago that Time Machine wasn't dropping oldest files. Even though there would be 100GB of available space, it would say Backup Failed. But at least it allowed me to record a new Time Machine copy on the same drive that failed. Now it won't even do that.
It appears to me that Apple has decided that it knows more than I do about my backup requirements and, in order to conform, I must spend money on new drives if I want Time Machine. I'll save the dough and, as you suggested, forego versioned backups and just have an extra CCC copy.
