I honestly don't think that MT is a bad QB. He is hopelessly average. He has some beautiful throws. Some athleticism. Some accuracy.
What you are describing is why he never will be a great QB. He can't make the proper reads and is slow making his progressions.
It does not excuse the horrid predictable play calling that hinders him far more than his above mentioned limitations.
With a competent OC he could be successful. Simply never great. And will not carry a team but could be a Neal O'Donnell.
Yeah like I said above, Trubisky would be fine (not good, fine) as a starter if you surrounded him with an incredibly talented lineup and a good scheme. The Steelers just absolutely do not have that.
Trubisky's biggest flaw is that he's horrible at reading the field and can't make his progressions fast enough. But if you had a good enough team where his first read would actually be a viable throwing option, instead of something the defense is set up to counter, you could actually make plays there.
The Steelers offense is highly predictable because they just flat out do not have a running game. You can criticize Canada as well, but I think the running game is the real main factor here. They can't build passing plays off of the run because their OL can't block and Harris isn't that good. That results in their play becoming extremely stagnant, where teams can set up to counter the medium pass (because that's all that Canada goes for) because they're not at risk of being burned by the running game by doing so.
I've said I don't know how good Canada is, not as a defense of him but more of a comment of ignorance. But I am confident when I say the lack of their running game causes their pass game to be crap. It doesn't matter if you have a pocket passer in Ben or a mobile guy in Trubisky, you need to have a running threat to complement the passing game.