It is common to run something like a RPi as a high level controller for things like: computer vision, AI, ROS, SLAM and high level user interfaces.
RPi zero does provide a small forma factor but is very light on processing power for some of the complex operation. A RPi4 has much more computing power especially if over clocked but has a much larger form factor which is also a disadvantage when trying to squeeze it into a robot.
The RPi compute module 4 is purposely design for use in embedded applications with form factor only slightly larger than a RPi4 but with more features than a RPi4 but doesn’t have anything broken out to connectors, rather everything is accessed via the high density headers.
Here is a very interesting small form factor break out board GitHub - harlab/CM4Ext_Nano: Smallest, yet feature rich baseboard for Raspberry Pi Compute Module 4
I am also wondering if someone in the know might be interested in designing a breakout board especially designed for using on robotics, I think aimed at the CM-550 controller??