Franz is all about handling messaging app while we handle all types of web applications. As for Rambox, indeed they are closer to what we do with a different type of interface though.

We do support self-hosted apps (any kind of custom apps) but as of today it requires a few weeks before it is added. In all honesty if that's a key factor for you, I would stay on Rambox and wait for us to have released the V.2 of custom applications before bothering to try it :)

Indeed PWA would be amazing especially for offline functionalities and push notifications.