In the meantime, I deleted everything out of the User Data Folder Except the Profiles themselves.
My Default Profile is now loading again and showing all the Extensions that were installed. Only thing is they're not really installed and have no icon (but maybe that's because of some of the older attempts with deleting stuff).
So now I can just go and just reinstall these.
The only thing that seems to be missing completely is the Chrom Web Store Button, but I guess that was an extension by Vivaldi themselves?
Now I just need to see how it behaves in the coming days and hopefully everything is going fine 🤞