You know the UCL competition is different from the English Premier League. In the Champions League you will see strong teams that are from different league, the champions league is not that easy to win. PSG has the likes of Messi, Neymar and Mbappe but yet they still find it difficult to win the Champions league. This same PSG is the best team in the French league, so I think Manchester City being the best team in the English Premier League does not give them a guarantee they must win champions league. I believe as winning the champions league is a priority for them they will surely have it anytime soon, because they are working towards it .
You are right, matches in the ucl are more stressful for every player any from team because they are under pressure to be able to win every game they play. for Manchester City they have good enough squad depth for this season plus the coach who has quite good experience in the champions league so this is the right time for Manchester City to get the UCL trophy.
apart from that there is chelsea where their performance in the premier league was not satisfactory but in the UCL they were able to perform better. So the big hope is for them to be able to maintain their performance for every match this season, whether in the UCL or PL, they must focus strongly on winning match after match.