WOW it turns out it IS possible but the method is EXTREMELY bad: you have to “copy” your m4r (ringtone in AAC format) files from Finder and then “paste” them into your connected device’s “Tones” section in iTunes. Drag and drop *does not work*. https://support.apple.com/en-ca/HT207955
now I just need to remember what the heck it was I wanted to add as a custom ringtone 🤔🤔🤔