2-7/8" oil field pipe works just fine here, only time I have used anything close to 3.5" is for gate posts in a corral.  We don't cap any of them and don't do anything to the bottom.  I have brush painted some with rustoleum, looks nice but is incredibly time consuming.  Did it more to cover the welds than anything.  Pounding works but if you have a skidsteer get a 3" auger or 4" if you go with the bigger pipe and drill a hole if at all possible, it is 10x faster and add a little quickcrete to make a much better post.  There is a lot of cheap oil field pipe in KS and OK, cheap enough I can drive all the way down there and get it and still come out ahead over buying "local".  Do have to be careful on the used stuff as some can be rusted very badly, if you can find a pipe testing facility they will have cleaned pipe that just didn't pass test to be used in a well again.

Went to pipe for same reasons, never wanted to have to deal with braces, corrals or anywhere else it can be used again.  We leased a place that had pipe line posts that were old steam engine boiler rods pushing 100 years old they were finally starting to rot off in spots and were a lot thinner than any pipe I have put in.  They actually curled and/or mushroomed at the top where they had been sledge hammered in.