Just because it's true and you believe it IRL doesn't mean your medieval ulta-conservative tyrant should believe it, too
You know I love you, Norrel, but medieval tyrants believed it all the time. They would get away with it if they could. They just couldn't, because sending the Church of Sartan packing is like a two on the scale of things that register as weighty decisions, while sending the Catholic Church packing was another matter.
Henry II comes to mind for an early Medieval King - 'would someone rid me of this troublesome priest.' And he was that way for the same reason that BM monarchs are that way: he didn't want people meddling in his business. Even worse for BM, religion doesn't bring in support or gold to the extent that it did in the middle ages, so your medieval King could at least count on a number of benefits from doing the Pope's bidding.
The medieval church was enormously corrupt and everybody knew it, down to the peasants who didn't know anything. They accepted it as a less bad alternative to another dark age. Heresy was very common, as were deals made between local bishops and local lords to enrich both at the expense of the Church (or vice versa). Which is why Henry II got his ass handed to him when he went up against the Pope more often than not.
The real trouble with making this happen in BM is that it would have to happen very quickly. Galiard isn't really anti-religion; he's just enough of a pragmatist that he sees inviting an organized church into his midst as trading away a significant chunk of his power in return for what exactly? Partially it's just that the Sartanist church is always its own worst enemy, but partially it's that Galiard knows his subjects will all get tended to anyway because all the religions are so happy to have followers that they'll take the table scraps they can get.
Crusader Kings models this very well, where you had holdings that belonged to whatever the local religion was, and making friends with them meant you got their tax instead of the church - and enabled you to make one of your 'own' bishops an anti-pope. Conservative tyrants like incentives. I agree that BM religion is not an analogue for the catholic church and I don't know that I'd want to make it one, but you could empower religion a bit more by making the priest game a little bit more about being a prince-bishop of some holding that non-priests can't have so that they can deliver the things medieval priests could deliver: gold, food, and even soldiers.
