Tattoos belong to all classes. Please don't try to make them a thaumaturge only thing. I'm pro-tattoo myself, but think they're too fun/thematic to belong to any one class. If anything, a general archetype would be better and probably outside the scope of this book (or not?!).
From a more mechanical perspective, Thaumaturges aren't casters so if they are getting 'relic' tattoos then all you've done is create a relic that doesn't require a hand or action economy to manipulate which sort of seems like a key balancing factor in Mark's design.