How to do a loop that bounces (pingpong) after hitting end?

Wrap in a new symbol.

Inside the wrapper, duplicate the original animation, covert it to frame by frame, remove first and last frame, reverse frames and append at the end of the original.

The wrapper set to Loop will ping-pong.

select the frames with the "to" motion.  in a different layer paste and reverse those frames for the "fro" motion.  position the fro motion to suit your needs.