View under the wooden ceiling

Even if it hurts, I would now rip it out down to the bare beams, then have the beams assessed. Then carry out a proper construction that matches the substance. I have also had very old beam layers with several "underlayers" made of the most diverse materials. I filled a container with that stuff. But now it is properly done. (well, that was already 20 years ago ...)

Thank you for the feedback! I just wanted to give an update: The ceiling in the hallway extends all the way down to the attic and will be newly insulated and closed from below. We are still considering whether it is necessary in the other rooms as well...

You won’t want the mess later; if you don’t do it now, it will either never get done, or if you realize it does have to be done, you will kick yourselves for not having done it right away.