I’m sorry that they’ve jacked up your costs, but not sorry to see it go (eventually) 
May I suggest an API first approach? If you provide a simple REST API just for configuring email accounts, a few of us (well, at least me) would be happy to use that instead of cPanel. That would allow for building a mobile client, web client, etc. as stand-alone efforts. Heck, you could probably crowd source some of that…
Heya,
I’m happy to tell you that’s exactly what we’re doing! I’m building a REST API with NestJS. It has an auto generated swagger definition, so you can generate client libraries for your own projects in many languages. My application is just handling auth and account permissions and passing most info over to WildDuck, a modern mailserver that is completely api controlled.
On top of the API I’m building an Angular app, which is starting to look quite nice if I say so myself. This should eventually replace cPanel and DirectAdmin.
I’m open sourcing both projects, you’ll be free to contribute or fork them. 