Trying to do hardware and software simultaneously from the start is going to kill this project. Pick one and get it right; let the lessons from that inform the next phase.

It looks like you're building a pretty generic device with an interesting software stack, so I'd start by making the software shine, and leveraging e.g. LineageOS to get an install base. Once you have feedback from a good swath real users you can look at making a bespoke device based on real-world needs.