LG Ultrawide 21:9, MBP 2017 no HiDIPI 2560x1080
With the help of Stephan Madrau, the creator of SwitchResX I have found a solution for my problem:

half of the non HiDPI resolution is the HiDPI resolution, so I am using SCALED 3840 x 1620 non HiDPI --&gt; 1920 x 810 HiDPI. It is a bit big.. but it is looking fresh and crisp.