My thought was to put my penny worth in to the original poster about trimming in a turn.
I don’t think there is any way to judge proper trim needed by looking at the control wheel. Level control wheel does not go hand in hand with what you learn on your second or third flight in basic training; Look out, apply aileron, rudder, slight back pressure, opposite aileron and release some rudder.. The amount likely varies with airspeed, bank angle, airframe. The only thing that tells you if you’re skidding/slipping is the turn coordinater and seat of the pants. It was easy to tell in a twin prop when your student did it right and when they were struggling with coordination.
About the flaps in turn, I think everyone will judge there and then if they feel they’re on top of it and ready to extend flaps. Turning or not. I guess it doesn’t happen too frequently these days someone is flying a single engine ILS in real life.