may I just ask you why you want to switch from an (even for SSDs) absolutely stable and fast filesystem like HFS+ to APFS?
I just don´t understand - because HFS+ runs extremely fast and absolutely secure on SSDs...
Space on HDDs and SSDs nowadays is also no more expensive - so if your filesystem will not be faster nor more stable nor much cheaper - why the hell taking the high risk facing really big problems using a new filesystem ?
Even if you want to "gain space" using APFS, is the little difference (if even existing and worth mentioning under real daily conditions) worth a higher risk to loose data, loss of time for going back and forth with the 2 systems, taking the risk of non-existing compatibility (as for Time Machine) and learning bitter things by the hard way?
It is just a question - I´d wait at least 1-2 years to be shure this "just works" ...