The problem is those players are also very rare. You're referring to the best of the best, for whom we would have to spend a lot of money on. Big money signings hasn't worked for us in the past. Zubimenbi was the other midfielder we were after but while he someone who can create from deep, he isn't as defensively strong as Ugarte.
How many midfielders today fit the category of being great defensively but also create? Probably only Rodri, every other great midfielder has a certain weakness that can be exploited. Kimmich is an elite creator but can't play as a lone number 6, same for Hakan and Frenkie. Paulhinha is a great defender but not a great creator. I suppose Caicedo could fit that type but he's quite young and inconsistent at the moment, even then we all saw how much he went for last season.
Two options are there, either you adjust your tactics to fit the weaknesses and strengths of your squad, something Klopp did really well with regards to his midfield (essentially removing the need for them to be creative by having TAA and Robertson bomb forward and be extra creators) or improve the players you currently. have so that they can become the players you want.
People forget who much Rodri struggled during his first season. I can remember posts from City fans here mocking a Gundogan Rodri midfield as being too open to counters, now any team would dream of having those two in the center. Rodri improved a lot in his years at City.
Ultimately, both Ugarte and Mainoo are young for midfielders so there is a lot of time for them to improve. It would just need the right manager and a lot of patience, both are things we are uncertain of at the moment.