I think topspin and sidespin serves are easier to learn but I think it is better if you would teach underspin serves first while developing their ball contact. Letting them feed underspin balls can also help develop their contact.
Defintely harder than Genesis 2 m. I think Genesis 2 M is only at 47 degrees. The Tango PS especially has a profound euro feel. The K1 Pro is almost similar to PS version.
It is like playing anti almost but more on loose non spinny balls. If they punch block then it has knuckle ball effect. Try looping with a close racket angle or drive the ball with more like a hitting stroke.