Would he be willing to work under Garrett?
I agree that he's an upgrade. It seems like they would have to completely abandon Garrett's antiquated, 1990 offensive constraints and let Kubiak install and call the entire offense to get him to come here.
I would prefer someone who would design and call an offense that plays to Dak's abilities. As well as a QB coach who will tighten up his mechanics and force him to focus on improving his accuracy.
When he was at Houston, Kubiak's QB coach was Karl Dorrell (former HC at UCLA). He is currently the Jets WR coach so I don't know if he would be available, but if Kubiak came, I'd hope he would bring Dorrell or his son, Klint, who is(was?) the Broncos QB Coach. You can't give Dak Prescott an unproven first timer and then wonder why he isn't improving. Kellen Moore can be re-assigned to the scouting department.