My issue is solved!
When all else had failed, MS Support asked if I was able to test my issue in a store that sells Surface devices, and fortunately I was. The store was kind enough to let me test all my setups and I tested these three scenario's:
- attach a new Surface Pro 7 via Docking station, using mini-DP connection to my BenQ 2420 monitor. This immediately caused the flickering issue
- attach my current Surface Pro 7 via Docking station, using mini-DP connection to a new in-store HP Pavilion QHD monitor. No flickering! So this meant conclusively that it was my configuration that was at fault.
- now I tried to hook up my Surface via Docking station, using an "active" ** HDMI -> mini-DP adapter to connect to my monitor with an HDMI cable. No flickering + high (2560 x 1440) resolution!
Hopefully this will help some of you achieve a working configuration.
Thank you, Microsoft Support for sticking with me on this issue!
** I had tried to use a normal ("passive") HDMI -> mini-DP connector before, and this gave me a non-flickering display, but the resolution maxed out at 1920 x 1080, so this did not give me my preferred situation.