I see what you mean, I think it all comes down to command like and you and I agree on. I think for some of those elite pitchers that you mentioned, yes it's a factor of the spin rate, but also the fact that they feature premium velocity, elite secondaries and excellent command of all their pitches that makes them get away with a middle-middle mistake every now and then. If Reid-Foley improves his command substantially to where both the fastball and slider are both consistent, then I think he too can avoid getting crushed and get away with a mistake or two; the fact is that he just doesn't really know where the ball is going a lot of the time, and this obviously needs to be improved.
As far as two pitch starters go, I can think of many of the game's elite and other good pitchers that feature such a mix: Archer, Lance McCullers (though his is curveball/fastball), Foltynewicz, Patrick Corbin, Luis Severino to name a few. There are varying factors to their success, but they can get by with pretty much a two-pitch mix with a rare third pitch mixed in to give hitters a different look.