This has taken such a long time. So many dozens of little things to manufacture and assemble and redesign and manufacture and assemble. But I finally have a tiny RC car that actually drives! I cut and filed a bunch of tiny steel and brass parts for the drivetrain and steering assemblies and they actually work. Not great, but they work. I immediately burned out two steering servos by driving them into mechanical interference. They got stuck against stops and instantly burned out or broke. So I implemented software limits to ensure the steering and wheels wouldn't collide with the chassis. Then I added in the drive motor and found my clearances were too tight in several places, so I redesigned the drivetrain housing several times and now that finally moves smoothly. Then I attempted to assemble all the moving parts on the baseplate and found more issues that were solved by several more redesigns. Then I attempted to add on the body/housing and found a few more issues that required more redesign. But at long last, everything fits together and everything moves fairly smoothly.
After completing the mechanical assembly, I realize my power electronics redesign was inadequate. I found I really nice, cheap, compact, easy to work with battery controller that allowed me to charge my 1S battery from USB-C 5V, and also boost my 1S voltage to 5V supply up to a few amps. And that was great when all the parts were working individually. But now that I'm actually driving the little car, I find that 5V is simply not enough for the drive motor. The car is pathetically slow, especially while turning. My previous tiny RC car drove the motor directly from a 2S battery and that was just about perfect. 5V is not enough. So my options are to either add in another boost converter to increase the motor voltage or to go back to a 2S battery. I bought some USB-C 2S battery controllers and I'm going to try those out. And I have 8V->5V converters left over from the previous tiny car. One of those strategies should work.
Next up I'm waiting for the latest PCB revisions to arrive. I consolidated the two USB ports into one. No more using a USB micro for programming and a USB-C for charging. Now it's one USB-C for both. Also I realized I should just make the vehicle PCB the same size as the Raspberry Pi Pico, since that's a size that can't be reduced. There's no reason anything should stick out beyond that outline.
At this point I have a completed tinyRC car! It's not great, but it all works! Lights! Four-wheel steering! Driving! Printed body! Remote control over bluetooth! Tons of things to improve in the next revision.