Great to know you are also working on this.
I think the problem with the center of mass could also be worse depending on the number of arms, for instance of the robot only has one arm, you would need to also balance out weight of it.
One thing I thought of is to change the placement of the arms closer to the center of the body during different speeds (the faster the robot moves the closer the arms are to the robot’s center of mass) or during deceleration.
I also had the idea of putting the center of mass in the back, you just have to be careful when moving the robot backward and then suddenly stopping, but the stopper part you mentioned is a good solution for prototyping to keep the robot from being damaged.
Have you guys tried using the Unity plugin to try to simulate placing Reachy on a mobile base to test multiple base configurations, speeds, and different terrain types/inclinations, objects held while navigating, etc. before creating a physical prototype of the mobile base? I’m thinking of doing this but I’m not sure how much fidelity in the virtual environment and test results I can expect from it to transfer over to the real world.
Unity’s new robotics hub looks like a possible solution, they also have an example of mobile robot control using ROS.