If you don’t need the dolly to be freerunning for realtime shots, you can also mount the servo instead of the third wheel:
In the video it drives with a steplength of 200ms.
I put a small rubber “wheel” on it. It’s a glue-on foot you can get at homedepots.
There is already a small PCB installed for the ATtiny 85.I found out the best tracks for it are plastic profiles like this L-profile.