If Onana had peak De Gea's shot stopping abilities, but was also useless with the ball, he'd probably be rated higher on here.
As soon as we start controlling our games, be that under ETH or someone else, like you said, Onana will be able to showcase his passing, and ball-playing qualities even more, whilst also facing way fewer shots.
I said this before, but if you swapped Ederson with Onana, I don't think City's overall game or results would be much different, whilst Ederson would concede a ton of goals here at United, just like Onana has, whilst also not having his strengths maximized (passing, on the ball abilities) like he has at City.