Thank you for you explanation about the mesh, I have a better understanding of how the explicit solver works now, thanks. I have changed the mesh as you suggested. Still, the smallest characteristic length is given by the elements on the impactor. Any less than 2 elements through the radius gives poor element quality. I could change to a square impactor, but I thought the sharp corner would be undesirable (maybe it doesn't matter). I used a bias while sweeping from top to bottom, to have smaller elements near the impact surface.
These are the resulting SRS plots, as you requested. They are recorded from a node 200 mm from the impact position (like in my first picture). The plate is 1m x 1m x 0.3 m (without symmetry). I used a Butterworth filter with lowpass 15000 Hz. May I ask why you wanted to see the SRS velocity and displacement plots? What are you looking for when observing them? I am only used to acceleration SRS.