Insulating a slanted wall / cavity before or behind insulation

Ah okay, that greatly simplifies the insulation. Then I would say you can draw from the full range, and in my opinion it doesn't matter if the insulation is carried out in different thicknesses.

When it comes to keeping the heat out, then, as others have already written, mass with the highest possible heat storage capacity has a clear advantage. If it is a garden shed, however, most of the heat could also enter through single-glazed/unshaded windows or the roof. However, you might also have to consider what is economically worthwhile for a garden shed. Multiplex would be one option, cement-bonded wood-based materials (chipboard), also known as Heraklith or Sauerkraut board, would be my first idea - possibly also double. If it is supposed to be from the inside. From the outside, the world is open to you ;)

As already mentioned, all amateur opinion.

On the one hand, the insulation should serve as light soundproofing and in winter as frost protection for plants to overwinter. If it then still manages to block a little bit of the direct heat in summer, that would be all the better, but that is not our primary purpose.

Thank you for your opinion, . The windows are triple-glazed, and the roof will also be insulated. Since plywood is currently unavailable (EU anti-dumping measures against Russia), we have to switch to pine plywood. We wanted wooden panels as cladding for aesthetic reasons, and therefore not chipboard or drywall with plaster/wallpaper.