Is there a way to create APFS snapshots on external drives?
I would like to figure out how to do this as well, but have yet to figure out how to do it either. I'm sure it has to be possible somehow because Carbon Copy Cloner can manage snapshots on external drives that you use as backup targets. If CCC can do it, then there must be a way. Until we figure out how to do it manually though, I let CCC do it.

You can't tell it to make a snapshot directly, you have to set up a backup task and let it run, but you should be able to get it to work for you. I got it to create snapshots for me by setting up a backup task with an empty folder as the source and by setting it to leave the existing contents of the target drive alone. That way it does nothing to the drive but still makes the snapshot (it makes one on the target drive before and after each backup).

Once you have a backup scheme that works for you, you can schedule tasks to run on a schedule, after certain events, or manually. You can even chain tasks together so one runs right after the last, so you don't have to guess how long one will take before the next should start (say one is set to 12:00 and the next at 1:00 - either the one will run long and two will be going at once, or it will run short and you'll have unnecessary down time). I love it. 

If you haven't already, I would definitely suggest you try it out. I've been using CCC to keep backups up my computer for years now. It is an amazing backup program and they keep adding great new features and tweaking old ones to make them better. Very worth the $40 (I think $30 for students and teachers) and they have a 30-day, full feature trial. You could try it out to see it can do what you are looking for.

Bonus. Here is an article about using snapshots with CCC, and here is another about APFS and CCC.