Why not use the same system that many radio controlled models use. Pulse width modulation. That would give you proportional control on each channel rather than just on/off control. If you only want on/off control of up to 8 channels you could use a UART each end of the link. It is many years since I last used UARTs and they were in a 40 pin DIL package. I don't know if they are still available as most microcontrollers have built in UARts and cost less than I remember paying for a UART IC.
Les.