I think things must be more complicated across the pond.

Here (AFAIK) we have a standard tow ball size ( I think it is 50mm) and the restricting features are "noseweight" and towing capacity of the car and thec capacity of the bar itself. So the detachable Westfalia towbar fitted to my E class estate (wagon) (which can take a "noseweight" of 85kg) has a maximum noseweight limit of 130kg (i.e the limiting factor is the car's noseweight capacity).

I bought a Atera Strada DL3 rear carrier with 4 bike option (because it slides back on rails - rather than tilt- to allow the dogs to have access to the rear of the car) which weighs 16kg, leaving 69kg for bikes.

Just back from 3 months in France and it coped well with two MTBs and two road bikes (one steel/one carbon). No effect on driving (the car self levels and has airmatic suspension anyway) Very stable, no excessive moving about. Fuel consumption went from 41 mpg to 35 mpg