You make some good points and I believe he is capable of being a productive player, that's a good point referencing Havertz but I'd also argue that Chelsea as a selling club seem to be a rule unto themselves with the value they seemingly are able to get when selling, i would not be surprised if something was uncovered later on as they sell 10x better than any other big club in the PL.
Havertz (If we are comparing) also was playing a decent level, Chelsea didn't say they want rid of him, the player didn't publicly ask for a new challenge and it's quite well thought of that Rashford has application/ attitude issues. The issue isn't "He went clubbing", it's him regularly doing things that don't look great, that regularly reference that football or Manchester United isn't his priority anymore.
I wouldn't be against a straight loan if we don't get the fee we want as I do think he would look good at a club like Barcelona as 90% of the teams they play are not at the level of a PL team and he will naturally look 10x better.
I don't think it's about solely stats but if you'd like to go down that road, let's look at this season and last and he has a goal or assist every 3 and that's the story of his career bar 2 half seasons where he exploded and showed ridiculous consistency, I would take Dibling from Southampton who they managed to sign for £21m so I think it's definitely possible to find someone who can give us a similar return especially if they are low on contract such as Jonathan David.
Anyway I don't mean to make this personal, I agree that we should get as much as we can for this player and we created a big problem by giving him a contract until 2028, if we wanted to pay him off and someone offered him £150k a year then at the moment he has 3.5 years of his £350k contract and he would only move if we paid off the difference. (350-150 = 200 x 52 x 3.5) is over £36m and if the club want's this £20m net figure for PSR that means we have to sell him for £56m and get him to agree a contract with another club on a lower fee.
It's not that in a perfect world he would be worth more, he would. On his day he's top class, but that does is too infrequent and with his other baggage (Wage, Attitude etc) the realistic side of me can't see us getting more for him, I probably would loan him out to a club I believe he will bang them in and revisit in the summer.