Are you leaving any gap between the planks (like a deck) or butting them tight (sort of like tongue and groove)?

Any ventilation in that space underneath or will the wood seal it more/less tight? Any inspection port-type thing?

When I bought my house, the home inspector wouldn't sign off my tiny back porch (maybe 6x6') since he couldn't see up underneath it to look for termites - it was completely boarded up. I'd take photos to show the absence of termite/critter damage while it's opened up.

x3 on trimming the rot off the columns, then a block or two underneath. Maybe one square, then one oversize round, just for interest. Do a search for 'column base' for ideas. If you're cutting a lot, maybe make it symmetrical top and bottom cap/base so you don't have a 6" tall stack of blocks under the column.

I think that code for the steps is that they be in equal increments +/- a fraction of an inch, so that each step is predictable. You may need custom stringers to finish the job right.

Also - galvanized or stainless hardware with the treated lumber, right? Common nails/screws will rust, stain and corrode away.