Is Vecna the Ultimate Big Bad in Stranger Things?

After finishing volume 2 of season 5 I’m left wondering how the mind flayer is going to fit into the finale of the show. Is it going to be revealed he is the ultimate big bad? Or are we going to see him at all?

I know the prequel play details Henry being teleported to dimension X when he touches Brenner’s stolen briefcase in the Nevada cave, and is then flayed and infected by the mind flayer. This is what gives him powers and further corrupts him mentally. He alludes to a “black monster” in volume 2 whilst manipulating the children, but I’m a little bummed we haven’t actually seen the mind flayer yet and I’m getting to the point where I’m doubting we will even see it again.

Or is it the Thessalhydra…