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2 metres deep should be plenty. Mine is about 900mm deep and 800 x 800 with quite a bit of steel in it then the top was raised about 100mm with the final top part more like 1.2metre square so there was a flange in concrete on top. That may also be helping to prevent movement.
I think what SB are saying is the bottom of the pier needs to be supported and not sticking in the air. I anchor bolted my pier to the slab and then I levelled it with packing pieces. Then I put sand/cement mix under the pier to pack it up.
You can get nonshrinking grout that is good for that purpose. I believe it can be quite liquid so you could flow it under the plate. Stone masons use it to grout big blocks of sandstone that are in place on a building so its good stuff. In my case sand/cement shoved under with a trowel and smoothed off worked really well and my pier is well supported.
It makes sense to ground the pier well as Brendan could tell you, it would be a waste of the strength of that deep pier if you allowed flex from the weak point which is where the pier attached to the pier slab.
That base plate needs to be well supported with no air gaps and lots of anchor bolts nice and tight. That's where the slots are handy as you need to know which way is the south celestial pole before you fix it down hard otherwise unless you have a rotating pier adapter you may not have your mount close enough to the south celestial pole to achieve polar alignment. So mark that out on your pier before you install anchor bolts.
Greg.
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