What saddle, bike, seatpost and specific tire model, size and pressure? Optimizing those can help.
From what you wrote, I am interpreting your fire pain to be soreness in the deeper tissues and not surface abrasion or irritation from salt, feces, etc. Skin irritation can be helped with a viscous lubricant. I used to use Lantiseptic but the last three times I ordered it, it was very thin and watery and did not last long. I am experimenting with others. When it is hot, I sweat and you do not want bacteria to enter thru your skin and want to minimize friction but it sounds like this is not your issue.
A very common problem is a too high saddle. I see it all the time on brevets. Too high of a saddle will result in excessive hip movement with the hinge point being the connection at the saddle. I would check that. Some degree of butt discomfort is common. I used to never have any saddle issues but I used to ride a lot more and was younger. Fitness comes into play. As a rider tires, they sit up more and more. They put out less power. These two factors place MORE weight on the derriere especially transients, the arms are not there to take some of the shock. So, it all goes into your ass.
The right saddle is one key. A leather saddle will mold to your bottom and distribute the forces over a large area. Some like them and some do not.
Congratulations on the SR completion!! Sorry this is not to helpful