Subsequent roof insulation

I want to add insulation to the roof of a single-family house from the 1960s. The roof structure is as follows from inside to outside

Gypsum, reed, boarding with air gap, rafters, 60 mm mineral wool between the rafters, air, flush boarding, several layers of roofing felt.

Since the roof is still watertight, I would prefer not to install improved insulation between the rafters. For that, one would senselessly have to open up the ceiling from the inside and then rebuild it. Also, covering the felt from the outside and removing the boarding makes little sense.

What do you think about applying 100 mm PUR boards on top of the intact roofing felt from the outside and sealing them again with roofing felt? I would re-board the edges and pay attention to thermal bridges.

You have to be a bit careful because of moisture.
Keyword: diffusion-open and diffusion-tight. Unfortunately, I can't tell you more about that here either, I'm not an expert.
If you do it wrong, your roof will end up molding massively.
Better to get advice from an expert; if it's just about the advice and you trust yourself to do the work, then it's hardly a big expense to worry about.

With the PUR panels, you should think in advance about soundproofing. Soundproofing from PUR or rigid foam insulation is less or basically not effective.
It depends on the respective situation (place of residence, heavily trafficked street in front of the door, etc.).
You still have the interior insulation, which will probably provide enough soundproofing, but it would be very annoying if it later turned out to be the wrong insulation material for you.