henno I think you misunderstood few things.
In the display settings - Don't use Built-in Display as Main display. Instead use the Dummy 16:10 as Main display. Built-in Display should mirror the dummy 16:10.
Built-In display brightness should be always 100%.
Adjust the dummy 16:10 display brightness according to your eye comfort. I find 65% brightness in dummy display is usable.
In the betterDisplay application - Set resolution of the dummy to 1920*1200. All other settings in your BetterDisplay should look similar to this below.
Display settings should look like below.
Display settings of the dummy should look like below.
MacOs version doesn't matter here. But sill for the reference pasting my os version here. Ventura 13.4.1 (c)