I've installed Zorin as dual-boot now. I noticed a heat-up after the successful installation and saw that the proprietary Nvidia driver was already installed although I actually wanted to do that manually.
However, I opened the Nvidia Server Settings and noticed that "Performance Mode" was pre-set so I switched to "On-Demand". Due to Zorin being installed I could actually try that setting out by re-starting the laptop.
And voilà: It seems to have solved the heat problem. The Nvidia temperature currently is at about 40°C which I find normal.
I will continue to monitor the temperatures closely for some time, but I am optimistic that everything is now in order.
Thank you all for your help!