The Brooks suggestion is a good one.
As for the seatpost suspensions, I would expect them to be designed for people of lesser weight, and therefore you may find they are almost bottomed out before you hit a bump.
Another thought, since you say your legs generally aren't tired - try getting more function out of them by standing up to let the bike float over bumps more often, and at the same time you will be giving your arse a rest.
Thought
#4 : I have completely abandoned tires narrower than 32mm - my current faves are 700X35mm and I will likely go wider as my expectation of speed goes down and expectation of comfort goes up - which makes a big difference to my posterior compared to the 25mm tires I used to use.