Also not recognized for Time Machine- USB APFS (Score 2)

Maybe I missed the warning, but I thought, why not convert my USB connected Time Machine drive to APFS... the conversion is allowed in Disk Utility, so I started it up. After a looonnnnggggg time (more than 6 hours, but less than 21 hours) it completed. And Time Machine says "Where is my backup disk?" It's right there, same name and everything, just APFS. OK, when checking disk selected, I see my disk a second time in the list, so I choose that. TM tells me that I need to erase the disk. Well, I really don't have anything much on this machine, so sure, let's erase it and start over. Only after it is erased, TM says it can't find the disk. NICE! So that's where that stands. Maybe I am having issues due to older hardware (the MacBook Pro still has an optical drive, for ghod's sake!) or something else, but I think that Time Machine is borked when it comes to APFS drives, at least USB ones (can't check any other's- or maybe I could partition the SSD and try to use the second partition for TM? hmm, something to try.