Lee,
Looking at the sketch from an engineer's perspective (no real pier building experience, but I know what's flexible and what's not...
) - Can you go any thicker on the baseplate?
A 5mm baseplate will be very flexible as you've drawn it (compared to the rest). I would recommend going to a 10mm or even a 12mm baseplate, and invest in some good non-shrink grout to go underneath it.
What you've got should still be way better than a tripod I reckon, but for the sake of a little thicker plate you could eliminate any chance of a low frequency resonance problem.
Al.