What's going on the outside and how are you capping it?

Here's a picture of the stone columns I built. The cap stones are 22" square. The tapered wood column will go from the cap stones to the porch header band.

From the picture, these are obviously structural columns. You should have had some sort of metal post base in the top of your stone columns.

You still don't say what you would prefer what the finished surface would be, other then wood. If you stick a PT post in to support the Porch roof, your options to fir it out are limitless. Will You paint the wood or leave it natural?

Unless you really get a really good grade of plywood (and the marine grade might be), plywood does not finish that well. In other words, unless you put a lot of coats of paint on, you will always be able to tell that it is plywood.

I would put a PT 4x4 inside to support the porch, so whatever is on the outside does not have to be load bearing. The final finish I want is smooth painted white. I am not so sure plywood will give me the finish I want. I am looking into PVC trim boards. That might work well. I did get a quote for pre-made columsn....$1200!! No way can I spend that.

Marine plywood differs from normal plywood because there are no voids on the plys inside,its made for stress all the plys are clear.Exposed to the weather it reacts the same as any other plywood.
I kinda like the idea of cement board or course you will have too tape the seams with fiberglass/mastic and do some flooting but man the things would never rot or have insect damage. John