Your design looks sound but you need a decentralized stabilizer.
The decentralized stabilizer is a mechanism that needs to be installed on the edge of the platform to make sure that is stays in the same direction as the pneumatic cylinder alows the platform to turn 360°. This must be quite strong as the forces on this are high.
Met vriendelijke groet,
I thought that was really nice for him to go so far as to give me advice on my design since he builds these professionally. I can see why they charge so much, there are a lot of specialized pieces which have added up to $2000 easily when all is said and done (let alone the custom software I will need to write).
The moral of the story is if they sell a large-bore non-rotating cylinder – BUY THAT. I know they have non-rotating cylinders but haven’t seen one that big. You could also tie two of them together (I think most of the non-rotating ones have 2 rods) so that there is a single U-joint at the top.