Cheap attic conversion - How to insulate, what to consider?

In an old farmhouse, the attic is to be insulated and converted. Currently, a horizontal cross battens are nailed on the vertical wooden beams from the outside, on which the tiles lie directly. To insulate properly in a professional manner, the roof would have to be completely covered and rebuilt, which is not an option due to cost reasons. Since the roof itself seems to be sealed, the following question arises for me: If I wedge insulating material (mineral wool, etc.) between the vertical wooden beams and screw/nail wooden panels to the wooden beams from the inside, what could happen in the worst case? The insulation material cannot fly away, since the cross battens plus tiles lie over it from the outside.

You mean cavity insulation between rafters, google it. It is among other things state of the art and basically feasible for DIYers. Our roof structure was like this 24 cm rafters or 24 cm insulation Foil (welded/glued) Counter battens Drywall No insulation above rafters necessary. For older and slimmer rafters, possibly consider insulation below rafters.

A seal must still be placed on the wool and for that the roof must be uncovered. Everything else is complete nonsense

A seal is still required on the wool, and for that the roof must be uncovered. Everything else is nonsense.

Spacers between the roof battens (on which the tiles rest) and then the appropriate membrane (there are various suppliers) glued between the rafters.
The roof does not need to come down. As I wrote, it is also professional and correct.

Currently, a horizontal batten is nailed to the vertical wooden beams from the outside, on which the tiles lie directly.

Apparently you are talking about rafters and roof battens here. Although practical skills are naturally more important than terminology and its correct use when renovating, your layperson's handicap seems to me to suggest that you seek more than just a little clarification of individual questions from an internet forum as support.

You mean insulation between the rafters, google it.
It is, among other things, state of the art and basically doable for DIYers.

It was carried out in my parents' house 20 years ago. No covering of the roof. Has worked without problems since then and can obviously, contrary to the suspicion of , be expertly executed successfully for years.

It seems to me your layman handicap suggests you to get more than just a little single-question enlightenment from an internet forum as support.

I fully agree with that.