What did you fly in 135, turboprops or piston? If steam gauges are more to your liking, that might be the way to go... 135 and then maybe 91 corporate (if you can find a long-term gig with better QOL than typical 135).
If you haven't flown old-school turboprops, it is a bit busier and more complex than typical piston planes but typically slower and with fewer techie bells and whistles than modern glass jets.
I would advise you to carefully assess your natural aptitude (flying is not worth getting killed over), but since you flew 135 I assume you're comfortable with complex planes in IFR.
It's tougher to learn when you're older, I've observed that as an instructor and experienced it myself. I've had success by living a very healthy lifestyle before and during training... plenty of cardio exercise, minimal crap food and booze, lots of veggies and brain food. You can literally feel the cognitive improvement after about one week of living right.
But if you're still in the game... flows, callouts, and FMS procedures is the meat of what you need to learn. Practice those your buddy (a must!) and maximize use of whatever FMS trainer is available.