Advanced Thaumaturgy Question

Advanced Thaumaturgy - 30% less cost of skills

I assume its not but i just want to be crystal about this, does this also apply to reservation?

Nah

Most likely no because on what we know in Path of Exile 1, anything that reduce reservation has to have specific keyword for it. "Reduce Mana Reservation" "Reduce X skill Reservation Cost" "X skill have less Mana Reservation" you get the point. I assume it would work just the same in Path of Exile 2.