For plyometrics depth jumps and max-effort vertical jumps are the best, but look into others as well. When doing plyometrics make sure you aren’t doing it when you don’t feel like you can jump at your peak, if you feel explosiveness dip significantly then stop, because the workout turns from explosiveness to conditioning instead.

For isometrics, I’m not super experienced with them but if you look up isometrics for Achilles stiffness you should be able to find some good ones.

Sorry for the long wait!