I finally managed to compile the startpos with a completely new additional faction, not a conversion of an existing faction.
First the usual stuff, I edited:
-factions
-factions_to_faction_group_junctions
-political_parties
-political_parties_effect_bundles
-faction_political_parties_junctions
-faction_banners
-faction_uniform_colours
-faction_civil_war_setups
then:
- start_pos_factions
- start_pos_regions
- start_pos_characters
- start_pos_character_to_settlements
- start_pos_land_units
As mentioned in other tutorials, I then copied the files from raw and working data to the Campaign folder in EmpireDesignData.
Now to the part I just figured out:
I created a *.pack with everything except the start_pos files, placed it in my data folder and selected it in my launcher.
Now I was able to compile the start_pos.
I can really believe nobody figured it out before, but I couldn't find anything.
Also, every tutorial I read states to deselect all mods. The solution was to do the opposite.
So I hope this is of help to somebody.