Been collecting parts for a lawnmower and had decided on two wheel drive with casters.
This is commonly called "zero turn".
For something this size, two Brushless outrunner motors with perhaps direct drive?
https://www.instructables.com/id/Brushl ... ing-Robot/
No need to make it a balance bot, but you could.
I am starting to see drivers now, some are servo type
Some are for robot arms.
https://hackaday.io/project/157812-3d-p ... t-actuator
Direct drive Stepper motors will have great torque, but might be top speed limited.
https://www.e3s-conferences.org/article ... _01028.pdf
Those yellow hobby motors will wear out.
The software is the hard part.