Stick to class II at first; guessing you can't roll those, so flip=swim right? Float bags and a decent helmet (like a WRSI) are standard equipment for whitewater. If you love it, get ww kayaks, and take a roll class. Or if you dont want to learn to roll, maybe duckies, a ww sit-on-top, raft or SUP might be your thing. Rent, demo, borrow. Rec kayaks are slowww on flatwater, but if you have nothing to compare them to, that may not matter to you
. They do have great primary stability (they "feel" stable).
I know an old time canoer who keeps on canoeing, even when offered touring kayaks that are twice as fast. Whatever floats your boat
P.S. I am not too familiar with CO; I recall some class II below Shoshone on the Colorado.. Utah has the Green River Daily; and some stretches of the Moab Daily would be easy and super scenic. Keep in mind that if you swim on a bigger river, and its just the two of you, there is a significant chance of losing your boat and/or paddle. Could practice swimming with a rec kayak + paddle on a lake, just to have an idea of how hard it is.