bolidJS

If you can connect to a different characteristic, and send the "commands" there, I think you should define a custom characteristic for controlling the car. NRF.setServices, and handle the onWrite event.
I imagine pretty similar to what you did previously.
And since payload for one command would be less than the packet size, the whole command arrives as one unit, command handling would be extremely simple, no need for buffering or anything like that. And not "polluting" the Nordic Uart with car control commands.