I'm currently running a copy of High Sierra 10.13.1 on an -OLD- Lacie firewire800 external drive.
The drive is initialized to HFS+ (NOT to APFS).
High Sierra runs "well enough". That is to say, boot times are quite slow (of course!), but once up-and-running, it does ok. It takes longer to load a -large- app such as Final Cut Pro X, but again, once running, it's ok.
I see NO advantages to trying to convert an external platter-based HDD to APFS...