I think the main thing is that it be a break away post. For years the DOT would mow off my mailbox on a 4x4 post very winter.
I picked up four Chevy 16" eight bolt rims a yard sale a couple of years ago for $20 for the four. I was going to weld them together for a mail box post but was told that here in Michigan the post can't be stronger than a 4 X 4 post or if someone hits it and gets hurt by it they can sue you. Still laying out in the scrap pile.
Like you I welded up some wheels to mount my mail box on. However i only used 2 ft re-rod stakes to anchor it.
When they would knock it over I would just pick it up and set it in the snow bank till spring.