FC3 aircraft are for sure easy just based on their simplified implementation. out of "DCS" level modules I'd say the F-5 honestly just because it doesn't have any fancy avionics outside the radar.
I wouldn't say I've mastered any personally...I've been fling the A-10C since the original beta days and I still forget stuff.
I played around well with F-5E. I love its mission flexibility and how good it is in ground support. I'm equally having similar experience with some other jets the same way you're dealing with A-10C.