It's going pretty well, I'm learning new things every day 
Last week, I've implemented the configuration of the Tag Panel fields and finally fixed an issue with UTF-16 encoding of ID3v2 fields, which only appears under macOS.
I've added a separate newsletter regarding Mp3tag for macOS at https://www.mp3tag.de/en/mac-osx.html — once it reaches alpha/beta stage, I'll invite via this newsletter.
I'm so glad to hear you're still working on the macOS version, because there really is no other program that offers what Mp3tag does.
I'm still on macOS Mojave purely because I don't want to leave behind my Wine version of Mp3tag!
I’m running it on Catalina... 
Only thing I’m really missing is lack of context menu integration.
how? I'm desperately waiting for a native 64bit release and have a bunch of 1500-ish files to tag...
It is still Wine underneath, making it all possible... 