"Would be great if someone with the issue could test: simply enable content search for all your apps - you won't lose a thing and may fix it."
It worked! I disabled the spotlight settings for all Apps individually - going from app to app within the Siri & Spotlight settings. And then I ENABLED it again on all spotlight apps - like other suggested here as well.
> So You do not need to reset all settings.
You might think spotlight should be slower then, but it actually became FASTER, nearly instant. So (re-)enabling spotlight search settings for -all- Apps basically FIXED the problem. So it seems to be a bug and not related to the amount of data that has to be indexed. I got 425 apps on my phone and 200GB of data. It took me 2 hours. Still, better than having to reset all settings.
Please, spread these news. Since the bug was driving me crazy (iPhone 14 Pro, iOS 16.1.)