It took ages, it was a ride, and I ended up finishing 2 tools not one - but transmission_4-mac seems to work just fine when built from transmission-mac: enable mac version by viraptor · Pull Request #500850 · NixOS/nixpkgs · GitHub
I’ll clean up tomorrow to add an entry point, but otherwise - success!
element-desktop and transmission seem to be fully functional now on darwin. (Once merged) I’ll check what else may be having issues soon. (Not sure if pinentry-mac is worth it since it’s archived/frozen already)