I think better stay with Brighton Albion and he get more minutes playing than joins with Manchester City or United because will get difficult with regular position there. In Brighton Albion he has more minutes playing on every match, he can shows his skill than have to be on the bench when joining with top teams such as Manchester City or Manchester United who has top attacking line. Moves to bigger teams not get guarantee with more minutes player or regular position, I think Kaoru Mitoma better stay with Brighton Albion for one or two years later until has consistency performance and get guarantee as regular player when joining with City or United. Many Japan's players loss their position when moving to bigger team such as Takumi Minamino and Wataru Endō with Liverpool.
Manchester United don't have a good attacking line, only Hojlund is good and he play in center forward position. Mitoma is a left winger, unfortunately Rashford is a left winger too, Ten Hag will not replace him because he know Rashford has a big name and he will get criticized to replace with Mitoma.
But I agree with you he will become a substitute player in Manchester City because every players in this team is top tier.
Cause Marcus Rashford is a big name means he shouldn't be dropped if he underperforms? Now, that would lead to his downfall. He should learn from Pep Guardiola.
Pep Guardiola dumped Raheem Sterling and Gabriel Jesus, even went to the extent of selling both players ( To Arsenal and Chelsea ) same season, he ended up winning the Premier League yet again.
Same way they have plans dropping Onana if he doesn't improves, same should be applied whenever Marcus Rashford underperforms.
Mitoma would be a good fit, he can bench both Marcus Rashford and Garnacho if they underperforms, no time to settle for less.