Yay, finally found it. I finally just started to move each . folder in my home away and see if I could make preview come back (e.g. mv .cache .cache.bak), restart dolphin and see. This brought me to ~/.local, then to ~/.local/share, finally to ~/.local/share/mime
Here I found some entries from back when I had installed this stupid WPS office package, which puts crap all over the system I feel. I had uninstalled it long ago but the configs were kept I suppose. I removed most of these without checking one-by-one, finally finding the line application/wps-office.pdf in ~/.local/share/mime/types to be the one to disable (but it could be others as well).
So in sum, if someone else encounters a similar issue, have a look at (re)moving ~/.local/share/mime/ or at least look for potential source of your problem there.
Edit: Thanks Zesko, this wasn’t the source of my problem in the end but you put me on the right track 