I'd go larger with the cement slab, at least 1M cube. depending on ground movement etc... you may find the ground will move through the seasons so going deeper reduces the chance of surface movement. Hiring a Dingo or similar with a drill attachment would make this job easier. Nothing wrong with going the two part pier, I have seen quite a few designed in this way, just so long as you have good contact between plates. I would use the same diameter tubing on bottom and top aswell, wont make much difference, but would give you a better footing. Also make sure the bottom half is hollow so you can fill the pier later, a hollow tube is really not that good, you will notice a hell of alot of difference if you fill the pier with foundry sand. Rumples, Peter and I have done this and it makes a great difference. The gussets look okay.
hope this gives you a few ideas.