Is there any way to negotiate down privately held student loans?

My husband has about $150k in student loan debt remaining. These were originally Fed Student loans that he refinanced. They are now Serviced by FirstMark and I believe the underlying loan is held by JP Morgan Chase. We have paid these loans down aggressively for years (they were nearly $700k), and it is so frustrating to see how much still remains.

I was reading other threads where people have successfully negotiated down medical debt and credit card debt by calling and asking for various different payment options, claiming a financial hardship, or offering to pay all cash up front.

I understand that student loans are different in many ways. My husband also consistently makes payments and these loans have never been in forbearance etc, so I am not sure the lender has any incentive to offer him anything like what I mentioned above, but I wanted to see if anyone has done this, and if so, how exactly?