But you do seem to get to host a https version of your app in case you need features locked behind secure context.

That can't be right. If they're hosting on a different DNS they have an absolute need to MITM ssl/tls traffic. Can't work otherwise.

So cloudflare sees your plaintext. Btw: tailscale does not (but ssl errors and warnings are unavoidable)

This is a crowded space with lots of solutions (oss and commercial)

https://github.com/anderspitman/awesome-tunneling