I'm not sure a midfield 3 of Mainoo, Neves and Mount/Bruno is ideal as far as height is concerned. And I think it also lacks quite a bit physically and athletically. There is a notion on here among certain posters who seem to be obsessed with 'press resistant' midfielders and completely ignore or are oblivious to what else is required to control the game out of possession. It's why players like Henderson and Wijnaldum were frowned upon but the press resistant Paul Pogba was lauded by us. So it's about creating a system of play where there's a good mix of players who have the ability to thrive in small spaces against the opponent's press whilst also having players who will be strong at controlling the game in large spaces against the transition in rest defense. We have some very good players in smaller spaces like Martinez, Shaw, Mainoo and even Dalot who has potential in that regard. And i'm sure we'll improve in that regard with whoever we bring in to replace Casemiro. Resisting the press starts with the GK and then the CBs, so before we focus on midfielders when it comes to resisting pressure, we need to focus on the positions behind the midfielders, because other teams have created strong build up phase capabilities which rely on their Gk, CBs and fullbacks and not their midfielders.
But IMO we as a team need to raise our level both physically and athletically which in-turn will allow us to control larger spaces when playing higher up the pitch. So without sacrificing technical ability on the ball, we need players at the base of the midfield who will ideally be strong duellers, large space controllers, and if possible improve our ability to defend set-pieces and also attack set-pieces in the opposition's box. I think both players mentioned by the OP (Onana & Wiefer) are two players who fit what i'm talking about. Wiefer I've mentioned on here a number of times since last year, and he potentially represents a low risk, high reward type of deal.
We have Mainoo as someone who we can develop as the deep lying midfielder to dictate the play in a roaming role but a player like Amadou Onana besides Mainoo does the job out of possession that Mainoo struggles with along with making us stronger on set pieces in both boxes. Onana's recovery pace, physicality and athleticism will provide us with something we severely lack out of possession. And he's more than capable in possession and in the highlights below he's being utilised as the first receiver in midfield against Man City. The only slight worry with him would be the injuries he picks up, which might be a reason not to sign him.
I also think the CB signings are linked to the criteria I've mentioned above about a new midfielder. And the clip below from Sky Sports where they have Jesse Marsche analysing the game is a great one because he's saying something about 'rest defense' which I've mention on here for a while which some posters seem to think is a buzz word. So it's important that whoever we sign as the CB(s) should also have the ability in small spaces (ideally), but we absolutely cannot ignore the physical and athletic element of the role because defending in a compact high block and allowing your fullbacks freedom of movement going forward will require your CBs to defend the channel in 1v1 situations. And for us to have a strong rest defense, it's absolutely imperative to have CBs who can defend on the front foot in a higher line. If the aim is to play the game in the opponent's half, then it's important to target CBs who are not only good box defenders but also good channel defenders. And for me, the most important attributes for a CB in such a proactive approach is to first have a level of technical quality in possession along with being physically and athletically dominant. That's more important than being aerially dominant imo.
Ideally we sign a CB with all the requisite attributes but I'd sacrifice aerial ability for pace and physicality along with technical ability in possession if idealism doesn’t materialize. You can make up for having a lack of height at CB by having strong aerial midfielders, fullbacks and even a striker. But you won't make up for having a aerially strong CB who is weak at defending the channel in a proactive attacking system.