I agree, I also feel Lavia is being overrated. I think Onana would be a bigger upgrade for us going off what I've read and heard from people who watch the player. And just watching Onana's highlights it's quite clear to see that his ability to receive passes on the half turn and carry the ball through midfield whilst showing good acceleration is impressive for a player that is 6'5. And people who watch him regularly have also said that he's always showing for the ball.
For a player that's 6'4/6'5, this level of acceleration, dribbling and passing isn't normal imo.
He's going to cost over £50m without a doubt and I think his profile is quite unique for a player of his size and technical ability. Of course there's things that will need ironing out at like his concentration on setpieces as
@SilentWitness said. But I think a player with Onana's profile helps us in and out of possession. And with a number of teams playing direct football in the EPL with the aim of winning second balls high into the opponent's half, its also important to have options in your team to combat that play style as well..