Add a new sort of body armor that makes a compromise between studded leather and crystal plate mail - less AC than crystal plate mail, but more than studded leather (so around 5 AC), weighs more than studded leather but less than crystal plate mail (so around 300-350 aum), and doesn’t hinder spellcasting at all. Possibly, “crystal scale mail”.

Possibly crystal plate mail’s base AC will have to be boosted, or its weight lowered, in order to avoid this new armor being just better (as it stands, a gain of about 2 AC in exchange for about 100 extra weight is something most players won’t take).