yes, a sit bones measurement is in order. Sounds like most of your weight is on your soft tissue instead of your bones. I think many of us "sit" differently when on a trainer which can also add
to the problem. I tend to find myself more upright and as I am stationary I somehow seem to change the curve of my back which can shift the weight to the soft tissue spot instead of the sit bones.

Wishing you luck! Hate the saddle hunt. I had a B17 imperial on my new bike and realized the "fenders" of the Brooks aren't for me so am ordering a new Rivet Pearl in black (matchy, matchy)
Love my Rivet!