re: Fence post

Costs are up, but I would think it would be cheaper than 6" x 6" posts (not sure where you would get those or what their price is). 1.5" square tube is something like $70ish for 24'. My guess is 3" square is $140 for a 24'. So about $45 a post (assuming you are going for a 6' privacy fence).

You can absolutely screw metal brackets with self tapping screws to the metal posts. It would be time consuming is the downfall. How far are you running the fence?

I hobby weld, and I got a steal of a deal on a ton of 4" square tube that is 1/4" wall. Absolutely overkill for what would be needed but it's what I had on hand and what I used. I think I got about 120 12' sticks for something like $8 a piece. A contractor I know ordered twice as much as he needed for a job and was looking to get rid of it and recover at least some of his money. Still have 90% of that inventory sitting in my backyard.