"do you agree that Mental Game is the biggest edge in the game once a player has reached a certain standard? "

I don't need "the biggest edge". My game is so full of flaws, both mental and poker-wise, that I can find improvement in so many ways. And cracking mental flaws that keep me from grinding more, or moving up stakes, has been and will continue to be an area where I can improve dramatically.

I actually think there's tons of research about "the mental game", though it might not be poker-specific. As an example, the concept of mindfulness is very well-documented, with benefits that span as wide as concentration to happiness to pain relief, and if someone doesn't want to learn about it until some big-time poker player espouses it, well, more power to us. Our ability to learn some of these strategies from other research fields, and apply them to our poker game before they become a thing every poker player does, is where we can have an advantage.

Would 100% agree with what you say about mindfulness and other areas of life having good content; it just feels neglected to me in this game, in comparison to definitive poker strategy conversation.
As all who have read, heard or studied Jared Tendler will know, he started in Golf, and it is scary how relatable everything is.
The underlying questions here are, how to do you study it, what techniques do you actually use, how do you get into the zone, etc etc.

We have some good articles here on the mental side of poker, in particular I would recommend you give @MattVIP's articles a read, he has some interesting ideas. I found his Mindful Breathing article helpful in poker and out, it really works for me.

Do you agree that Mental Game is the biggest edge in the game once a player has reached a certain standard?

I would not agree.

My mental game was always pretty darn good and I never played above my bankroll, my weakness was my poker game.

I have friends who have awesome poker minds but are total degens and have several times played above their stakes and lost a lot of money.

It's person dependent. Imagine having a scale, one side is skill the other side mindset, people have different amounts on either side.

Mindset is humans biggest problem - whatever we feel we do and we react to that. #nuffsaid