This sounds very similar to what I am going through now.

It isn't specific to the ischial tuberosity, but everything else sounds very familiar.

Apart from breaking my ankle 12 years ago, I have lived a pain-free existence on and off the bike. But after going (slowly) from 200 lbs to 165 lbs, I'm now suffering a lot of pain and weakness. It is mostly in my right leg/hip, but I don't think it is the joint. I think it is mainly sarcopenia. I am currently 62.

After 10 years on a nice Brooks, I can't tolerate it. I finally found a saddle I can manage: Ergon All Road. It isn't perfect, but it keeps me off the suicide hotline.

Sorry I have nothing to offer but comisseration. It is also really screwing up my sleep, which then makes it worse the next day.