Apart from his performance not optimal, his salary is also a burden for the club so no wonder they want him gone

There are many players who are on the bench at every club just to anticipate injuries to main players but this situation is not for Rashford due to budget waste. I agree with you, Rashford should pave the way for the Saudi Pro League there he will get high salary and spotlight but his career will probably end there because Saudi pro League is the last destination for top europan players. There are actually better options like moving to Serie A but the salary is certainly lower than the previous club.
Perhaps Rashford will eventually move to Saudi Arabia. His performance continues to decline, and no team is willing to pay more for him. Serie A teams cannot afford to pay Rashford 325K a week salary, but for some teams from the Saudi Pro League, that is an easy thing. I think most likely Barcelona will not will not make Rashford move permanent after his loan period is over.
Manchester United may have to start thinking about finding a new club willing to accommodate Rashford, a club from the MLS might be another option besides the Saudi Pro League. In terms of age, Rashford is actually in his prime, and perhaps in the future, he will be able to rediscover his best form.