Hello Zimm:
Have you thought about a moderate shock absorbing seatpost rather than a heavily padded saddle? I used to use a Thudbuster SL on my Sala Fargo to reduce some of the bite from washboards. I think the longest day was 85 miles of loaded touring & I felt fine. Currently I'm running the stock saddles that came on my 2017 Salsa Mukluk and Cutthroat. Those are WTB Volt Comps. They have more padding than I prefer, but work fine for touring saddles. A couple of weeks ago I took the Mukluk on a 2 day bikepack that ended up being about 120 miles of slightly to unmaintained gravel, two-track, and 20 miles of cross country down a remote canyon with more rock gardens than I planned on - the seat was fine.
When I was a roadie, my favorite saddles were Sella Italia SLR's. Almost no padding, but extremely comfortable as the shell flexed slightly between the nose and the rear rails. When I road mountain bikes I'd switch to a Sella Italia SLR XP which had a bit of padding. When we were younger my wife and I lived in Fort Collins, CO for several years and would usually take the winters off from cycling which meant that we had to break our butts in every spring. The saddle that was perfect for centuries in the fall was painful in the spring & we used to tell each other that it took 1,000 miles to get our butts back. If a saddle still felt poor after a 1,000 miles we'd think of changing it. I think there are two main considerations - are your "sit bones" on the saddle? Is the saddle too broad for your crotch? If the back of the saddle is too narrow then your sit bones may not be supported and too much of your weight will be supported by your crotch. That leads to chafing, saddle sores, and general misery. If your sit bones are supported, but the saddle is too broad for your crotch you may rub the insides of your upper thighs against the saddle and get some chafing.
A couple of other things that I find helpful on consecutive high mileage days - good cycling shorts with a "chamois" that's not too thick. I always wear a clean pair of cycling shorts each day (wash the previous day's in the morning and let them dry on your seat pack or panniers throughout the day). Some sort of a chamois lubricant - I usually use Chamois ButtR but I prefer Assos Chamois Cream if I'm in the saddle for more than 6 or 7 hours.
It sounds like you might have your bike set up for an exaggerated upright position? That can provide some relief for your hands and neck, but really puts a lot of weight onto your rear end. If you can strengthen your lower back and neck enough to use a more forward leaning position it will pay off in the long run - less windage and less problems with seats.
Howard L. Snell