Some more findings:
For testing purposes I installed the current Intel Graphic driver 27.20.100.8476 from the intel.com site on the SB3/13". After that the system immediately overheats - even if you don't run any specific application. So don't try that - It's even hard to remove the drivers after a cold boot before the system shuts down. That would kind of explain the reason for overheating conditions during graphics intense video meetings.
It also fuels the qualified guess that the Iris Plus drivers are key to the solution - my current (somewhat) working version is 26.20.100.7873 from 2/14/2020.
Probably due to the overheating condition of the graphics the main processor speed got decreased so significantly - the locking behavior at 0,19GHz seems to be fixed with the July firmware.
However, in certain scenarios the notebook still overheats - especially when using multi-monitor setups.
Maybe some of you could check their versions of the Iris Plus Driver to compare.