Save data to connected bangle.js on custom page

I think the current approach to write the whole Bangle.js app as a template string is not really suitable for complex apps

Yes, totally. In fact I just chose that as a quick example but I think for newer stuff we're just sending a config JSON and then having the app itself as a separate file (which you can reference from apps.json as normal).

have you any plans to add the communication with a Bangle.js like for the interface page?

Is there a reason for not adding an interface.html file as well? The idea of the customizer is to get called when the app is uploaded.

As I understand it, right now in order to start the customizer you have to have deleted your app first (apart from when updating) so if the app loader was set up right everything from your app should already have been deleted?