Since it sounds like you don’t have much concern over what you fly, but rather how you fly and maintaining good health, here’s my two cents: You’ll have more control over diet and exercise in the 135 world than 121. I know it sounds crazy, but hear me out. The reason I say that is you’ll have influence on the hotel choices more than the contracted vendors for whichever 121 you’d be with. In my experience, most 135s will say “Looks like you’ll be in ABC tonight… do you want the Hilton, Marriott, Wyndham option … ?” Then you can do a quick search to find proper nutrition nearby, whether it is Whole Foods, Wegmans, Publix, Ralph’s or whatever grocery store chain is in the region. Far better choices than the Holiday Inn Express abeam the runway with a Waffle House, In-N-Out, or McDonald’s in the parking lot. Plus you’ll have a crew car, or expense ground transportation (Lyft, Uber or cabs). The same strategy can help you find lodging near preferred gyms.
Sure, 135 presents some anxiety due to the randomness of where you’ll be each day, but it isn’t as bad as some make it out to be. Sure you might end up in Manhattan, KS or Manhattan, NY by way of Teterboro, but wherever you end up - it is the company’s problem to get you home. You’ll never have to stress out about commuting again.