I stumbled across the following post
[SOLVED] Bad quality when printing PETG
and have changed the settings to this effect
Now the print looks extremely good and I have only 1-2 imperfections! These come when material sets up or detaches and adheres again in another place and thus the subsequent layers then become unclean. Presumably, it is fundamentally difficult to realize a full-surface filling with a print of this size without something not running perfectly in one area.
I will still screw on the temperature and test the PETG material from Sunlu.