Take this as my own personal head canon and nothing more but IMO Thor (MCU, I haven’t read the comics) is the only one ever permanently worthy to wield the hammer.
Cap was worthy partially because of the circumstances. I don’t think Civil War era Cap would be deemed worthy just like cast away Thor wasn’t. By that same logic I think Tony could have wielded Mjolnir briefly during the fight against Thanos especially given that he was willing to sacrifice everything. I think very shortly after the battle he would no longer be able to lift the hammer.
Cap still carrying Mjolnir to return it to Asgard is a bit of a hole in my theory but if the hammer is sentient then it knows it must return to it’s timeline so maybe I can fudge that a bit.