My TOS symptoms are under control now and my major issue is occipital neck pains and headaches.
I wonder if anyone can suggest something.

I had headaches for 20+ years since college which progressively got worse and 10 years or so ago the neck pains started. At first these would come and go, but now I have them more or less daily. Muscle spasms on the back of the neck seem to be the cause of the pain.

Regular stretches and trigger point massage are somewhat helpful, but sometimes trigger really bad headaches and don't have any lasting effect.

The doctors only suggested over-the-counter painkillers which do help with the headaches although after taking them for many years I developed stomach problems (from NSAIDs) and tylenol addiction so I stopped taking them all unless I happen to have an unusually bad headache.
The really bad ones put me out of commission for one or two days. I get sensitive to light and sounds, nauseated etc. Thankfully, these only happen once every few months.

I was hoping regular posture exercises will bring some relief, but I am yet to see any improvement. Sometimes symptoms get better, but then flare up again. So I don't know how to proceed.

Any success resolving the neck/headache issues?