I have a steel pier made from junk yard materials. The pier tube is 12" dia, 3/8" thick, 3.5' tall with a 16" dia, 1/2" thick bottom flange and a 1/2" thick top mounting ring. When mounted on top of a concrete pier base, it rang like the Liberty Bell when struck. These frequencies of oscillations would probably interfere with any video AP work by blurring some of the super-fine detail you are trying to capture. For visual or long-exposure AP, I don't think your eyes could tell and long-exposures are going to average everything in those frequencies anyways.
The vibrations are not a problem after filling with sand. Our climate is extremely dry so I haven't seen or expect to have any problems with rust. You could always drop in a few bags of dessicant beads and cover the top with a rubber membrane if you wanted to try to prevent moisture.
I think your plan is sound. You can always put the leveling plate on top of your pier -- weld a square piece of steel with a hole cut in the middle to inject your sand. Put three holes in a triangular shape to mount three bolts. And fix another plate over this one to accept the bolts with washers and nuts to allow leveling.
If you decide later on to go with a base plate above the conrete pour, that shouldn't be too tough -- cut the pipe a little bit above the ground and weld a mounting plate with bolts to the top of it.
Depending on how much concrete you are set into, you might have some shift over time. You'll have less shift if the center of mass of the entire pier + telescope is below ground level.