Thorn slinger gives bonus point to other physical skills, but not to Thorn totem. Other items work, for example if I equip +2 helm, I get 2 skill points.
All skills are level 20, after I equip Thorn slinger, I can spend an additional point into Spriggan Form or Entangling Roots, but not into Thorn totem. As soon as I snend a point, I see a common visual bug, all skill are displayed as level 20, despite being 21. But for Thorn totem I cannot spend a point into it, so this is not just visual bug.
Thorn totem has physical tag, it should work
I don’t have cold conversion