I have a square pillar finished with cedar shakes. It is secured at the top of the pillar. The wooden front porch deck is free from the pillar, so as it rises and falls, the pillar sometimes loses contact with the deck and is then only suspended and sways. I want to build a slip joint, so it looks like a molding around the cedar shakes of the pillar but is really attached to the deck floor. Wondering if this will create a situation for rot down the road. So cedar shakes, cedar 2x6 porch deck floor, and slip joint/collar will be cedar. I thought of making this a 2-step with the bottom being approx 1/2 maybe 3/4 inch away from shakes and the top tighter against the shakes to prevent the pillar from swaying. Am I over thinking this? Will the cedar alone be the rot preventer?

You can buy PVC strips at HD /LOWES.use these to help support the column and put small SS screws through the column and PVC into the deck. Then you can put an ornamental trim piece on. The PVC won't rot.