@sequoia I think I’d look further into what could be done about your pain. Here’s a very good article about what sounds similar to your situation:
https://www.uofmhealth.org/conditions-treatments/surgery/chronic-pain-after-mastectomy-and-chest-surgery#:~:text=Nerve%20Injury%20Contributes%20to%20Post%2DMastectomy%20Pain&text=These%20nerve%20injuries%20can%20lead,%2Dcontinuity%2C%20and%20scar%20compression.
“Once nerve injury is identified as the source for the pain, surgical removal of the small, offending nerves can be performed in a straightforward outpatient operation. Small incisions on the flank (directly under the armpit) or just to the side of the breastbone or the spine are used to gain access to small nerves that come out to the skin from between the ribs. The damaged nerves are excised, with the anticipation of resolution of the pain. The side effect is numbness in the area. Most patients welcome this numbness as a welcome trade-off.”

If the surgeon says that insurance won’t pay for this - I’d suggest that you request that the surgeon provide the insurance company with a statement that outlines WHY you need the surgery. Number one issue the insurance might pay - is to keep you off opioids or even high levels of nsaids in the future. Although you might not be taking these now, I believe there is a high percentage of people who do and as you get older you may want the pain relief. The other issue insurance co might pay is that it is restricting your ability to exercise in ways that are healthy (core muscle exercises probably cause you pain?). If these, or other issues, are affecting your life and health you should discuss it with the doctor and, at least, get it in your records. Having a record of actual functioning and pain may help you in the future if you decide not to do anything now, but who knows what the future may bring?

Nothing to do with mastectomy. Everything to do with chemo. I was tempted to stop the chemo because of growing numbness in my hands and feet, but Covid was rampant at the time and I was keen to get out of there. I have tried acupuncture but it doesn't help.