I don't think the margins would be that good after fuel and maintenance of the 5.6 vehicles. The margins would be
marginally better with the V6, but the real profit maker would have to be a hybrid version - but then some consideration would have to be paid to luggage handling. The hybrids have about 3/4 the trunk space volume of the conventional ones.
If you focused on longer distances, such as airport trips and those were mainly highway routes; maybe the V8 would be a worthy contender to the non-hybrid V6. You'd still have a considerably higher maintenance cost in the long run.
Client comfort & entertainment
I find the back seats of most cars to be boring, but that's because I'm a driver first, not merely a traveler - my very subjective opinion.
I assume the rear of the Y51 would be extremely boring due to the non-existent creature comforts or doodads to play with back there.
What I'm saying is that after paying for one executive ride in an M37 or M56, I'd be left wondering if the experience was worth it over say, riding in a Passat or Kia Optima. But it has relatively little road noise.
For ride and comfort, the sport-designated versions of these vehicles are a little harsh (including the AWD sport like your M56xS), and isn't exactly what I would want coming off a 4-hour flight.
The base models are way more comfortable; in general the passenger seats are more comfortable than being in the driver seat.
Maybe none of these matter to your current situation, and neither would they to most other passengers. After all, when I travelled a lot for work, all I cared for was a clean car and relatively quiet ride when I needed to rest. I got all that from very many different economy cars - again, for most cars the rear seats are more comfortable than front ones.
You know your individual market better, and I bet there exists a calculator for such decisions: "how much more money can I make by upgrading to a larger luxury car and taking executive passengers?"
For example this article from 2015 -
https://www.hyrecar.com/blog/uberx-vs-uberblack/. You just have to factor in your maintenance & operation costs that far exceed that of a Hyundai Elantra.
Godspeed on your career decision, and may your decision work in favor of your health.
* When I travelled, on most of my longer trips with odd-hour arrivals & departures, I would get older people as Uber drivers. Retirees, people still employed, people with their own businesses, stay-at-home parents etc... I like talking to people & sharing stories. For the most part, they said Uber was still profitable for them. I particularly enjoyed a 2 hour ride talking to a guy operating as an LLC.