With heathen tolerance and reduced nationalism you won't have unrest anyway. There's not even a real reason to convert those lands right away thanks to the religious unity from Humanism. As I said the CB isn't that important anymore and the "Restore the Pentarchy" event fires quite early. Which means you can take at least Rome almost right away.
If anything Religious is overrated, not Humanism.
You will have a base tolerance of heathens of 0, 1 if you take a policy. You will have nationalism all the same, though reduced in duration. Even with -2 unrest, this is nowhere NEAR religious until you convert, but you won't be able to convert quickly.
Once you have converted, then humanist gives more -rr than religious, but only then. In TF provinces, however, your modifiers will overpower the unrest regardless.
I believe you are underestimating the utility of the superiority religious CB. Also note that religious is part of the religious/diplomatic/aristocratic truce break trifecta if one wants to go that route (and BYZ is an excellent candidate to do so).