EDIT: Could solve it after several hours. 40mm/sec and 10% power worked. Not perfectly but ok.
I just switched from the standard K40 controller with K40 whisperer to MiniGRBL with Lightburn.
Now all the engraved texts look bold.
If tried a lot of different settings and heights, so it shouldnt be the focus. I also cleaned the lense and realigned the last mirror.
Could it be software wise? Lines seem to have the same width as before. Also the text gets displayed just fine in lightburn.
Frontplates on the left side were made with k40 wisperer and the old controller.