You might want to read my blog post at
http://www.shirt-pocket.com/blog (specifically,
http://www.shirt-pocket.com/news_on_the_march) for a comment about copying, which isn't really faster.
To use a drive with both HFS+ and APFS, create a partition for the APFS container and then make that APFS. You can create as many volumes as you want in the container - they all share the same pool of space. You'll then want to back up to a volume in that container.