Seems like my question is as simple as challenging. I've found this page in the API Reference which demonstrates how you can disable default map behaviour.
Disable all zooming on the view | ArcGIS API for JavaScript 4.15
The functionality which we have to achieve in this project is to make the map "zoomable" only by CTRL + Mousewheel. I've disabled the "mousewheel only" functionality as explained in the link above. But how can I add a certain behaviour so that the map is zoomable again, but only by pressing CTRL in combination with the mousewheel.
I'm using JS API 4.15
Thanks in advance for your help! 