I was looking at the RAW export of the default Joplin notes and see the metadata is stored as tags at the end of the MD files. Since my notes are really simple (basically all markdown and few uses of tags) it may be pretty straightforward to parse and update the files that way.
Would you still recommend using the API over attempting a RAW conversion though?
It seems there are lots of ENEX to ___ conversions out there, but not the other way around.