Defending the title is not as easy as we say, even though Manchester City is very much closer to the potential for a trophy. But regarding the Champions League it feels quite difficult, it's not just about the ideal squad. At a time when several players who have helped Manchester City win trophies are now moving clubs, the game system will change slightly, even though options have been prepared. Picking up the UCL trophy next season is like rolling the dice. Therefore this competition is difficult to maintain, only a few clubs have won it successively and even then with a fairly long process.
In the last decade, only Real Madrid has been able to defend the Champions League title in 3 consecutive seasons, this is truly an extraordinary achievement. Not a single team has been able to match Real Madrid's current achievements even though they are good teams in their respective domestic leagues.
Manchester City have the opportunity to make great history in the Champions League if they successfully defend the title next season. But I agree with your opinion that defending the title is not as easy as say. Pep Guardiola certainly has enough understanding of the condition of his team, but he can't be 100% sure to secure the title even if he has a quality team.