Don’t convert your Time Machine volume from HFS+ to APFS
New base technologies in macOS High Sierra have allowed to improve the basic functions of Mac. But unfortunately, people still have problems with the limitations of the new Apple file system format (APFS) required for SSD.
Glenn Fleishman studied the problems with the limitations of the new format. He concluded that you do not need to update the disks manually for APFS. And also there is no advantage to using APFS on hard disks, and the file system is not ready.
If you are interested, then you can ask a question and maybe answer it.