The CDC (charge-discharge controller) of many BMS (battery management system) devices can already communicate the charge state. My home built e-bike uses a one-wire protocol, newer ones use a serial protocol. Even the plugs have become standardized these days. There are so many manufacturers selling components and kits, and 99.9% of them interoperate with components and kits from other manufacturers. Pick the bike computer you like, the form factor of the battery pack, size of battery pack (they all use 18650 cells anyway), BMS, CDC, wiring harness, motor. It is no more complicated today, and actually far easier so long as you size the motor to the bike frame and the battery to the motor, than assembling a desktop computer.