Here's an update on my adventure. I have been able to talk to an actual person from one of the spring companies. they had no clue, much less a solution. I have emails out to 2 companies. I tried the inquiry button at Brownells and actually got a response from a person, who referred me to another person. And then there was nothing but a blank screen.
I do have some possibilities. Wilson has a flat wire spring, but for 5" bbl. I remember seeing a David Tubbs video about flat wire springs for AR-15s. He was showing how the spring gained strength as it compressed, and then reduced power as it expands. But I don't know if it applies to pistols, nor to what extent.
Wolff has a variable power spring, and Sprinco has a progressive recoil spring. But I'll have to get in touch with them to find out how much the power changes. And neither are designed for a 6" bbl.
Thanks for your replies, and I'll keep you posted.