I don't really see how this makes a hardware wallet obsolete. A phone can always fail/brick/die/break/whatever. A hardware wallet never ever connects to the internet and never can. It will always go through a computer, the keys can be generated by it's seed and it's security is a lot better then some app on some phone (think of Trezor's pincode input). I would like to know what kind of steps you have taken to ensure security on the cold wallet phone (except disabling all communication).

I would prefer Trezor instead of Bither, but oh well, that is my personal opinion and subject to change. I do wish you all the best and good luck with this project!