LandCruiser temperature issues

Incorrect temperature readings and slow warm-up times definitely suggest a cooling system issue. A restricted cooling grille or radiator blockage can indeed cause these symptoms. Is the 'check engine' light constantly illuminated, or does it come and go? Have you checked the fuse for the engine cooling system? If there is a fuse and it's defective, that would explain the totality of the symptoms.

You mentioned a fuse for the engine cooling system...I didn't even know that was a thing! Where would I typically find that, and is it something I could check myself without special tools?

Yes, it’s quite easy to check yourself. The fuse box is typically located in the engine compartment or under the dashboard. Consult your LandCruiser's owner's manual for the exact location and the fuse diagram. A visual inspection should reveal if it's blown (the little wire inside will be broken). However, considering all the symptoms, it might be a bit more complex. I suggest you bring it to a workshop.

I appreciate the help! Turns out it WAS a defective fuse related to the cooling system. 52€ later and everything's back to normal! Thanks a bunch for pointing me in the right direction.