I don’t quite understand tiddlyhost’s JSON endpoint, if it is standard nodejs wiki HTTP API, then fetching plugin will be just GET its title, and you get a tiddler JSON.
In CPL, we support create plugin using Gatha, and simply fill the tiddlyhost URL. And CPL auto extract plugin JSON from HTML wiki, and available for download (using nodejs API inside Github action, which is invisible to users, and implementation is very simple, because nodejs wiki already provide these features).
BTW, download & upgrade plugin from 10+ different plugin library is not a good user experience. On user side, still need a centuralized entry.