Insulating a slanted wall / cavity before or behind insulation

But maybe a pro will come by

Sounds okay. That should solve the problem.

If it is actually not going to be used at all in winter, it will not have any heating.

Slanted wall: insulate from the outside and do not straighten. May be a character feature if applicable. Multiplex inside is okay if it stays dry enough inside. Sounds a bit risky.

Is actually not going to be used at all in winter, will also have no heating.

Why insulate at all?

With summer use, less sauna...

There is less sauna effect during summer use...

I honestly don't think so.
Thermal protection mainly comes from mass and phase shift.
A bit of glass wool behind a wooden facade is probably only useful in the very short term.