The gunners should focus on winning EPL first and if they have achieved that, they can think about winning UCL which I don't see that happening soon only if they can change their mentality and improve their performance up to the standard of strong clubs that are winning UCL.
I strongly agree with your current thinking, because Arsenal should focus on the EPL trophy first for them to get, so that the champion mentality possessed by the players can increase rapidly, besides that Arsenal needs to continue to improve their squad so that they have enough squad depth so that they can continue to compete and maintain their consistency in the fight for the title both in the domestic league and the Champions League later, Although currently Arsenal is starting to be able to compete in the Premier League, the mentality they have is not enough to compete in the Champions League and the proof is when last season Munich was in a very bad condition but they could not beat even though Arsenal previous performance was better than Munich in the League.
I don't know why it has being like that, is it that they always lose focus on the matches or they are not just lucky in the league. A lot must be done if Arsenal wants to win UCL. Arteta is a good coach and I hope that he doesn't make any mistake next season and win their domestic league. As long as Real Madrid exists, it will be difficult for weak teams like Arsenal to win UCL.
Arsenal main problem at the moment is none other than the mentality and consistency of their squad which is often down when facing important matches, Arsenal lost the opportunity to win the Premier League this season actually not because Manchester City is better than Arsenal, but actually Arsenal often lose points in crucial matches even though they only face teams weaker than them such as when losing to Aston Villa and Fulham last season, To be able to win the Premier League and even the Champions League, of course, Arteta needs to improve his squad mentality.