Lot of sandpaper and elbow grease.

I'd probably try to use a stripper for the polyurethane and then go from there.

You might consider, after first assuring that there is absolutely no metal embedded in the post, using a very sharp jack plane, or if available taking it to a shop with a large industrial planer to remove the stained surface layer first, then stain remover sandpaper to clean up the rest.

I'm not as well versed as rustic furniture building and finishing aficionados here but commoners I run with call wood that has weathered unusually long resulting in soft rings deteriorating faster than alternating hard rings resulting in a rubboard pattern. In we'ins shops we use a course wire brush on a 4.5 angle grinder to remove urethane and stain while recreating weathered pattern of barnwood. Sand blasting does a better job on flat boards when done by an experienced operator. Sandblasting might damage carvings on your piece.