Saddle, bars, and fit
It sounds like you have done lots of long distance rides. So I’m assuming you ride regularly in between the brevets to keep up your conditioning, strength, and endurance. Therefore the soreness sounds like an issue of saddle and saddle position. That plus the hand soreness to me sounds like your fit is off. However, base level conditioning is still essential even with the perfect components and perfect fit. What bike are you riding? Some bikes by design are harder for some riders to do long endurance rides.
Personally, I have ridden 2 Brooks leather saddles (B-17 for about 10 years, B-15 for 2 years) without a cut-out. I now prefer the Cambium-17 with cut-out (about 4-5 years, including PBP 2019). I’ve only briefly ridden the C17 without a cut-out but knew immediately it was not for me. The relief it provides for soft tissue is significant. For me the C17 with cut-out is the only choice. If you are have narrow hips you might use the c-15.
With a performance bike saddle and bike shorts with padding I do not wear underwear. Underwear gets in the way and causes chaffing and sores. This is something experienced cyclists take for granted but people fairly new to cycling don’t often know this detail. I prefer higher quality chamois. They tend to be thinner and cost more, but they are made of better padding that resists compression longer and do much better on long rides than squishy thick cheap chamois.
To minimize hand and wrist fatigue I move my hands around quite a bit unless I’m on the front of a paceline or on very rough roads. On my own I move to the flats, the ramps, the drops, the hoods. Every cm of a good road bar should be comfortable and this technique helps stave off fatigue. I have Nitto Noodle handlebars which have room on the ramps. If I ride only on the hoods I get fatigue and soreness and eventually numbness. I also use gloves with no padding and only one layer of bar tape. Gloves with padding
and extra bar tape actually lead to more numbness on long rides for me.
The biggest improvements and the key that unlocked very long distances for me was starting with a good saddle, good handlebars, and then finally a successful bike fit. Before the bike fit my knee and lack of conditioning prevented me from doing more than 400k. After the fit and taking conditioning/training more seriously I was able to complete SR series and PBP. I have found core strengthening and stretching to be helpful with neck/wrist/hands as well. I follow vinyasa yoga videos online every now and then for guided stretching and core strengthening.