Not taken offensively, no worries
In general I agree and I think Havertz has moved at the perfect time. Others like Tah have even missed the right moments to leave.
Thing is, Brandt wasn't ready for the next step before Bosz took over. And then he played like he never did before. You say he showed how good he can be but I don't fully agree. Had he showed what he did in his short period under Bosz for a full season, whole Europe would've been after him. That was hands down the best performance over 17 games I've seen from any Leverkusen player. Imagine a full season of that. He probably would've ended with 10-15 goals and 15-20 assists as a CM. He would've established himself in that role in public perception and I doubt that his next club would've played him out of position as much as Dortmund do. And he would've probably found his spot in the national team as well.
That's also why I think the comparison with Sancho isn't really a good one. Sancho has two seasons with world class scorers and a third one at age 17 that's still pretty good. He's proven his quality for years at this point. Brandt has done so for half a season. The Brandt before that was still a good player but not good enough for Dortmund.