Thanks for your tips and insights! Adding additional cooling fans or changing the thermal pads isn't really an option since the PC is brand new. Installed MSI Afterburner and started testing...

- Core clock -200 doesn't work since the LHR tuning needs a fixed/locked core clock to work correct
- Lowering the memory clock lowered the memory temp but killed the hash rate
- Lowering the power limit doesn't work since the LHR tuning bumps into it
Also see: https://github.com/Lolliedieb/lolMiner-releases/wiki/Nvidia-LHR-and-Kernel-Mode

I ended up with something else that more or less works:
- Set a fixed core clock at 900 via lolMiner itself (--cclk 900)
- Made a custom fan profile with MSI Afterburner including higher fan speeds
I now have slightly better memory temps of 98-102°C which seems good enough since I only mine 5-10 hours a day.
Screenshot: https://imgur.com/a/o9FprFb

Greets.