US and Australian units are different for insulation, but in my terms the insulation around the best ducts is R1.5, but good ceiling insulation is R6. 4x difference

Interesting. The code for duct insulation in unconditioned spaces in the US is R8, and they're pushing for higher currently.

They also recently undid a long held myth about ducts buried in insulation leading to condensation issues. Most areas now allow ducts to be buried as long as you have R19 above and below, or R30 above. When I just installed a ducted mini for 3 upstairs bedrooms, I surrounded my R8 flex duct with R30 insulation.

lol - it's not a myth. It's physics: https://www.youtube.com/watch?v=Vufih-WN5R4

Fwiw, context of this video is metal duct work. You’re not going to see R-8 flex duct buried in R-30 begin to sweat when the AC is running. In terms of controlling the dew point, the duct is no different than the conditioned space at that point.

Like you said, it’s physics.

Over insulation introduces new problems like condensation (which causes mold and other nastiness)

https://www.youtube.com/watch?v=Vufih-WN5R4

Your statement is quite context dependent, and does not apply to most modern residential scenarios. “Over insulating” a flex duct is not going to cause condensation. Metal duct work in an unconditioned space can cause condensation. Wrapping it in (often thin) fiberglass insulation can increase condensation in certain conditions. Improperly vented attics (and/or air leakage from conditioned space) with metal ducts can cause issues.