Take these things with a grain of salt, but here are some models that attempt to assess trade value:
http://nbasense.com/draft-pick-trade-value/4/jacob-goldstein-4
There are 4 versions of the model, here is what it would take to get #6 according to these charts:
Goldstein 2017: 6 for 9 + 42
Pelton 2017: 6 for 9 + 34
Pelton 2015: 6 for 9 + 50
Restifo 2016: 6 for 9 + 30
These are EV centric calculations, so not a true trade value.....but yeah I think they underestimate the cost from moving up to 6.....but it is a warning shot to show that the difference in outcome between 6 and 9 is not as big as you may think.
Some relevant recent draft day trades:
2020: Jarrett Culver (#6) FOR Cam Johnson (#11) + Dario Saric
2019: Deandre Hunter (#4) + Jordan Bone ($57) + Solomon Hill FOR Jaxson Hayes (#8) + NAW (#17) + two future second round picks
2018: Luka Doncic (#3) FOR Trae Young (#5) + future top 5 protected first round pick
Honestly, I think #6 for #9+#28+stuff (some seconds?) would not be out of the ordinary. It will depend on how much ORL wants who they can get at #6 versus #9.
To get to #4, it would probably take #9+#16+future first (not sure #28 is good enough).