Thanks for the educational responses. I've been practicing. As I mentioned in my second post in this thread, I watched a Youtuber do an IFR flight from KFOK to KGON (Long Island to Groton), and he did it without autopilot (but also without ATC). So I replicated his flight -- IFR, many low clouds, no auto-pilot -- except I kept the ATC on.
I managed the first couple legs surprisingly well, but ATC did not give me the ILS approach I was expecting, and I couldn't find the waypoint they assigned for me on Skyvector, so I had to ask them for vectors several times. I eventually found the waypoint and landed safely. It was very satisfying.
But I need a lot more practice. I tried again, this time from Cincinnati to a nearby municipal airport, and all went well -- until I reached down to fuss with the mixture, took too long, and put the plane in a spiral that caused too much stress on the aircraft. (I had that realism setting on, which is all well and good, except that it cost me a Flight Sim Economy fare!) End of flight.
Also, I can trim the aircraft to hold its altitude pretty well now. But horizontal trim is another matter. No matter what I do, the aircraft wants to roll slightly right or left. I try adding aileron trim, but that just delays an inevitable roll one way or the other. Maybe I should add more of a deadzone to my Saitek yoke?