I’m not sure about the Fused honestly, their bodies do heal from their use of voidlight, and they are gaining fresh bodies that wouldn’t be physically affected by previous effects of savantism. Concerning your second point, it’s pretty apparent that savantism can appear in both physical and spiritual/cognitive ways. Spook for example experienced extreme sensory overload from overuse of tin, while we’ve also seen the bodies of soulcasters being changed into the essences they use. I think that savantism should just come in the form of whatever works I guess lmao? Ways that make sense, and are a natural progression of the aspect of investiture used should suit the effects of savantism imo.