A year ago I "finished" a hobby project to build a remote control Zoox vehicle. That was 1/25 scale and both the vehicle (management doesn't like it being called a "car") and the controller used Raspberry Pi Picos and NRF24 radios. When Raspberry Pi added Bluetooth support to the Pico SDK, I figured it was time to upgrade. And also decrease everything to 1/36 scale because employees got 1/36 scale toy vehicles and I wanted mine to be the same size as those.
So I finally got around to digging some example code out of the SDK and cobbling together a remote control system. It works perfectly and it's about the same level of complexity as the NRF24 version. The only difference is that the NRF24s could be called by the user anytime the user wanted to update them, whereas the bluetooth stack ticks via a timer and calls a user-specified callback function. There might be a way to transmit irregularly as needed, but I don't actually need that so I just used the demo code I found.
At this point I have the exact same software functionality that I had in the previous version. I can control velocity, steering angle, drive direction, and steering mode from a single joystick. So far there are no other software changes, aside from a bit of cleanup.
For mechanical, of course I've redesigned everything smaller than before. I have a resin printer now, and that makes it much easier to print extra tiny parts for the drivetrain. I also realized I can just buy tiny universal joints and differentials from places like Alibaba, so I'm no longer attempting to print or machine those parts myself.
For electrical, I realized I don't need to open cases and unplug the batteries to recharge them. I can just buy USB-C battery controller boards for $1/each that will charge my batteries from 5V input and regulate their output up to 5VDC, so I don't need to build any of that functionality myself. Also, I always wanted to use the tiny and inexpensive joysticks from Nintendo Switch Joycons, and I found the connectors required to use those, so that decreases the size of the entire controller by quite a bit.
Since the software didn't change, it all works already on the new vehicle. The electrical and mechanical will take some time to redesign and re-complete. The drivetrain is so tiny ...