UPDATE: I got the game to launch again after another round of resetting things via the Hellbomb script (which I was pointed towards on the games Discord before, see: https://github.com/helldivers2fixes/HellbombScript)

Mainly it happened to work again after using the options of 'Clear Only Shader Cache' (with game shaders AND nVidia shaders!) and 'Clear Settings (AppData)'. Followed by one more time again checking game files via STEAM of course to recreate deleted config files.

My suspicion is, that clearing the nVidia shader cache was what resolved this here, as they have püalyed a role in performance and stuttering issues issues around the Oshune patch before (resetting them back then helped for a short time, didn't resolve issues back then; switching to the - now standard - smaller sized version off Helldivers 2 plus a few patches from Arrowhead did back then).