This is great and looking forward to all things you will do in the topre world
1. HHKB Pro 2 is a great choice
2. Bluetooth + USB 3.1 sounds great
3. Willing to pay 60-70-80$ on the final product
At the moment for HHKB's and FC660c/980c we have hasu and yang controllers that satisfy the programmable needs.
After the HHKB Pro 2, realforce tkl's would be a great option, even a limited novatouch run, as those are loved by the community but don't get any programmable options ATM.
For multipurpose I use a hasu-usb to usb , but the need for programmable topre TKL's is very much out there.
The big improvement here I find it to be the custom designing of topre pcb's, that enables a new era, where custom topre keyboards can be made.
Ex: Custom CNC alu funky case (let's say alice style) + topre pcs with BT&VIA + Niz 2019 sliders + well tuned Costar stabs = love