My initial intention was to additionally lower temperature of power pre-regulator by adding "thermal bridge" between bottom PCB side and post-regulator heatsink (for Q4 power mosfet). That will affect cooling fan operation since it could work with lower speed (and less audible noise) to manage the same temperature.

I found that use of conductive thermal pad e.g. aluminum block with PCB side insulated is a really bad choice since it will drastically increase switching noise!

If you'd like to improve cooling you can use a piece of 5 mm thick silicone thermal pad mounted. Channel temperature that is measured with NTC1 can be lowered in that way significantly. With e.g. 20 mm wide pad it’s possible to achieve 6-7 oC temperature drop on the max. load!
Search on eBay, AliExpress, etc. for e.g. 100mm x100mm x 5mm GPU RAM IC Chip Cooler Conductive Silicone Thermal Pad. It should be less then 10 USD.