It sounds like the player might think that you cannot roleplay with a competent character. Or that a competent character automatically equals a perfect, never-a-mistake-made, no-flaw character. Or that tragedy is the only way to create story. And perhaps it is an overreaction to earlier games where everyone was hyper-focused on kill X and loot Y.
But, beyond the conversations you've already had and the above posts, perhaps a talk with them to ply for more of a middle ground and middle path approach, where he can still be not optimal and have some character flaws (That are great stories to see how they adapt, learn, and grow over time! It's what the classics are often about, that character development) without automatically going against every single grain. And perhaps some examples from Youtube/etc would be useful as well, to demonstrate it. (I can't help you on which ones are good examples of which, though, as I've never watched any livestreamed play.)
I'm known in my group as the 'method actor one', the player who gets subsumed in his characters so much that my patterns of speech and mannerisms change, and I'll come with good backstories and characterizations. But I still create very effective characters, and that hasn't gotten in the way of creating interesting, vibrant, and moving stories.
Hope it gets resolved and great for you all!