I also think that Marcus Rashford will likely move to Barcelona especially because it looks Manchester United is ready to let him go. You made good points about how Barcelona needs natural left winger like him which makes transfer look like good fit for their team under Hansi Flick. Since Barcelona directors are reportedly talking to his agent it looks like move could really happen. It will be interesting to see if he actually transfers and how he plays if he does.
Manchester United have let Marcus Rashford to find a new club and they no longer hold back because of his attitude as a Manchester United player no longer respect the club.
Barcelona's failure to recruit Nico Williams made them interested in opening opportunities to recruit Marcus Rashford but in this case it might only be realized as a loan player because Marcus Rashford is still quite expensive.
This recruitment is also less ideal because Barcelona has a player who is in the same position as Marcus Rashford so that competition will appear.
Plus the high salary of Marcus Rashford so that it will make Barcelona a little difficult unless Marcus Rashford himself is willing to receive a lower salary because he previously said he only wanted to join Barcelona.