I'm working my way up to being able to wear the Ursine heavy witcher armor in the hopes of being able to stop spending most of my stamina on casting Quen and use some more interesting signs, however even when fighting weak enemies in medium armor Geralt gets stun-locked easily as soon as Quen breaks or runs out. Am I going to be able to effectively use heavy armor without having Quen on all the time, or will Geralt still require Quen to not be stun-locked by enemies?
No, heavy armor will reduce the damage you take (due to higher resistances, not because of a higher armor value, e.g. Grandmaster Ursine has 44% pierce resistance, Grandmaster Griffin 35%). But no matter what, you are not supposed to tank hits of enemies, you are supposed to parry or evade attacks, avoid taking damage whenever possible; that's simply how the game is designed. Quen allows you to circumvent this somewhat, but can create a dull experience.
On high difficulties, you will take a lot of damage when hit. Heavy armor will allow you to survive one or two more hits compared to medium, that's it. It also won't do anything to prevent a stunlock. On top of that, stamina regenerates slower in heavy armor.