Sloping roof not insulated. Is subsequent insulation possible?

Hello everyone,
I have an older house where unfortunately the roof slopes are not insulated. That means there is no insulation between the rafters. However, everything has already been covered on the inside with these plaster carrier boards and plastered accordingly. I have now taken the trouble to tear everything down on one slope, then put in insulation, foil on top, drywall, fill, sand, textured plaster, paint. Unfortunately, all that took a very long time in total because I can only work on it after work.
Since all the other walls are actually plastered really well on the inside, I would like to solve it differently for the other slopes if possible.
Now my questions:

Is there an insulating material that I can pour/blow into the cavity afterwards? What about the vapor barrier then?
The space between the boards of the roofing felt and the interior cladding is visible from the attic. About 16 cm in height would need to be filled with this material if possible.

There is that loose fill granulate for floors, but how does that work with the vapor barrier? I would be forever grateful if you could help me out here.

Huge thanks already for all the answers.

Cellulose comes to mind. It also releases the moisture again. Alternatively, you could inquire at a company, they then blow it out.

If it is really roofing felt under the tiles, I would rather go for 14cm for moisture removal.