I find Manchester City's early season experience a bit confusing. Despite having a strong squad across all aspects and a great coach like Pep, their performance hasn't improved. So, there are things beyond technical aspects that Pep needs to improve, perhaps tactics or player chemistry.
The team looks strong but with what Guardiola have built and achieved in the past I don't think this squad is any way near the quality of the past and that is just the reason why they struggled last season and are still struggling again now. To build a solid team as that is almost near impossible now but they can at least perform averagely but I don't expect them to dominate the league again as they did before. Pep needs to change so many things and re build the team as though he just joined a new club that way he will be able to transform the team to a promising team again.
You made great points about Donnarumma and Manchester City. You are right that Donnarumma is top goalkeeper and his youth and ability to adapt quickly would make him perfect fit for coach Pep Guardiola. When it comes to Manchester City it is frustrating to see them struggle despite having great team and world class coach. Problem likely is not just about their skills. I think their can be low team chemistry or issues with their tactics. Pep will have to figure out what is wrong to get them back on track.
Only one person cannot solve the teams problems, yeah if he adapts quickly he will definitely help the team but I still think the defense is not strong enough and the goalkeeper will get exposed easily. If nothing is done they will still suffer the same fate as last season and things can even get worse for them.