Regarding the notes and questions I had above, the last open issue is now solved (see following) therefore I gonna mark this post as solved.
I wasn’t able to add the iOS platform. The problem was linked somehow to my local cocoapods which wasn’t up-to-date
pod repo update
should have fix the issue, but somehow for me it doesn’t work, this was throwing the error
[!] Failed to connect to GitHub to update the CocoaPods/Specs specs repo - Please check if you are offline, or that GitHub is down
but I was then able to update it like following
sudo rm -fr ~/.cocoapods/repos/master
pod setup
also just to be sure I reinstalled pod again
sudo gem install cocoapods
then I was finally succesfully able to add the iOS platform
npx cap add ios