I’d recommend getting a good, full carbon fibre post in your frame’s natural size. Don’t bother with the carbon shaft/alloy bonded head models, they don’t provide any real relief.
bigger the seat tube / seat post diameter, generally more harsh. the skinnier 27.2mm setups definitely flex more.
I ran a Thomson on my first road bike, a spare from my old MTB, and soon changed it when I realized how harsh it actually was.
Easton’s EC-70 is available relatively cheaply from usual on-line retailers and is well known for providing a degree of comfort / damping. they also do a EC-90 but is the same idea but with a carbon top clamp, and a big hike in price.
I just bought one off CRC for my new Giant XTC Advanced 29’er hardtail which had a very stout 30.9 Giant aluminium alloy post.
Replaced the Giant stock post with a Easton EC70 in 30.9mm and noticeable difference to comfort.