Fully offline is great, because students get the full experience, and access it repeatedly without cost -- so lots of pre-downloaded material. Downloaded HTML and still images, or pdfs are great for mobile phone viewing
Then chat apps such as email or messaging systems are especially great, because when the network drops, nothing is lost and the conversation can resume later
I would suggest teachers do this, possibly with telegram channels or similar. That way the class can have a group to participate in and share problems. Could this be the kind of confrencing you're after. It is as low bandwidth as it gets, resilient against network failure, and allows real time chat in a group...
Often more directivity is wanted though.... One option might be teachers recording audio-visual videos (their voice + preferably blank background slideshow as the visual). Which I think you're already doing
Another option is voip. Which is already heavily optimized for low bandwidth. Something like [1] or the team chat options computer gamers use. Onething to consider is if it is possible to record the audio session for later download -- since it is frustrating for students who can't get in reception range or if the audio drops out -- but a download might let them catch what they missed. If that would be useful to you, I don't know. But I suggest browsing the voip options, since there are many
I'm a programmer. But none of my suggestions involve programming, or creating new apps. There are good reasons for this. 1) It is a lot of work. 2) Students would need to download and learn them. 3)
Existing apps they know fix some of these problems really well
One final suggestion. Try zoom, with video disabled, and the shared screen view for teacher to draw on. If this doesn't work for you, then probably no custom app is going to either
[1] https://en.m.wikipedia.org/wiki/Mumble_(software)