Arsenal is not as strong as City and that is why they should focus more on winning EPL first, if they have won EPL twice maybe that will strengthen and improve their mentality to focus on being champions because they have already tasted winning a title, I think that will motivate them.
You have some points in what you’ve said about Arsenal and Manchester City but that is not entirely the way it would always be. You don’t have to be a champions in your domestic league before you can be champion in the champions league. I know that the champions league is more intense than the premier league and all teams needs to put in their best effort before they can even dream of winning the champions league. But if they can be able to to put in their best effort into just one competition, they can come out victorious. Like focusing just on the champions league without giving much effort to the domestic can make them win the league ahead of others. They just need to use the right players and apply the right technique to make it easier for them to prevail.