“/” should be fine. I believe it’s what the ID3 spec calls for. I think Plex also normally honors the semicolon as well. I use “/” and it seems to work well. My OS is also linux (Mint 20), so we have some common ground.
I tried putting a hyphen in one of the genres, but Plex handled it without a problem.
So, the challenge is figuring out why it isn’t working for you. My first inclination is to make sure Plex is fully configured to prefer embedded tags.
I’d also like to see a screenshot of your tag editor showing one of your examples.
