I may also change my bed to aluminum/kapton heater. Not a fan of the PCB type expecially at 12x12 which bows and warps so easy. If you go with a SSR make sure its quality like a Crydom SSR . The cheap Fotek style dont work to well and cant handle PIDs.

As for the plunging im not sure why its doing it .I think its something in the mesh doing it as it only plunges down a few mm before homing the x and y but i dont have anything in my end code in S3D relating to Z at all. Could it have something to do with the below line in the mesh leveling code

#define MESH_HOME_SEARCH_Z 2

What does that do?