I don't disagree with a lot of your points and selling players is obviously a big part of Dortmund's model, but I think your analysis here leans on that argument a little too heavily. While Bellingham's potential future value obviously will have been a factor in signing him, the idea that he was given more minutes to maximise his value at the (implied) expense of trying to maximise the club's season success almost paints a bit of a caricature of how people on here think the the club operates.
I agree that Bellingham's first season wasn't that extraordinarily special if you don't take his age into consideration. And to some extent I also think it's a possibility that the heights he could reach might be overhyped simply due to the fact that he's already so good at such a young age, as it's not out of the question that he's closer to his potential peak than other players his age, as opposed to his ceiling being astronomically high. Not saying I don't rate him, quite the opposite. I think he's already a fantastic player with room to improve yet, but that doesn't mean he'll become the greatest midfielder ever, to put it hyperbolically.
However, I don't think Bellingham got so many minutes straight away in his first season because he was prioritised over other players to speed up his development. Dortmund just didn't have many better options available that season, but not for lack of trying to have a deeper squad with more options. While he obviously was signed with the intention of being an immediate. option, part of the reason was simply injuries and other players underperforming or declining. In theory they had plenty of options in midfield with Witsel, Dahoud, Delaney, Can, or even Brandt or Reus depending on the formation. Witsel was still the first name in midfield until his injury halfway through Dahoud and Brandt on the other hand were pretty disappointing that season. Delaney never fully recovered his form after his injury as hoped, and Can was frequently needed in defence due to lack of defenders, especially when playing a back three. Also Can turned out to be a bit underwhelming, which might not surprise some but he was still an expensive signing for them and came with higher expectations.
So yeah, Bellingham impressed right away considering his age, but he didn't have to dazzle, given form and injuries of his competition. But I wouldn't say it was planned for him to face so little competition, or that he played because flipping him for profit takes precedence over their team's ambitions. They thought they had a good team together, but that's not how it turned out. And for all this talk of Dortmund's business model being about selling players, and Bellingham's inevitable move sooner or later, I think people misunderstand that model to mean they wouldn't love hang to hang on to him if that's what he wanted for whatever reason. It's not going to happen because clearly Dortmund can't match other clubs' contract offers or ambitions, but pretty sure they'd love nothing more. After all even being a selling club doesn't mean you have to sell your most valuable player, when there are other players they could sell to generate some cash if need be. But that's a pipe dream I suppose