The laws have changed massively with new federal DOT stuff. In most states, you can waltz into any Hertz, Ryder, or Enterprise and rent a 26' box truck with air brakes and 26,000 GVW regardless of what your license is. The theater rents one 6 times a year, and it doesn't matter which one of us rents it - me with a CDL, or the boss with a regular class C.
The air brake endorsement disappeared from my CDL even though I was trained and it used to be on my license. It went away because you no longer need an endorsement.
In most states, the 16+ passenger rule only applies if you are operating the vehicle commercially and it is registered/titled/insured commercially. Hence why it's called a commercial driver's license and not a large-vehicle license or multi-passenger license. In PA, I could purchase a 72-passenger school bus, leave the seats in, and transport my 52 family members on a plain old class C license.
Read up on the new laws. They are nothing like what they were when I got a CDL. As a very basic rule of thumb, as long as the vehicle is under 26,000 gvw and not being operated commercially, it doesn't need a CDL. In some states, the CDL isn't even required for vehicles over 26,000. I can walk into a Marathon coach showroom, plop down $1.5M, and drive away in a new 45' Prevost motor coach with marble floors and a solid gold toilet, but it's a non-commercial RV. Sure, it's built on a "charter bus" but it's not for hire.
The CDL is less about how to safely operate a large vehicle and more about knowing the commercial DOT laws. The base CDL training teaches you things like railroad crossing laws, when you have to stop a weigh stations, weight limit restrictions, and max width/length/height. The passenger endorsement isn't about how to handle the vehicle with 50 people on it, it's knowing that you are responsible for only transporting X lbs of flammable liquids in the luggage compartment, or knowing that it's against the law to ask why a passenger has a service dog. If you're not operating commercially, those laws don't apply in the same way and they therefore don't require you to get the license.