Is travel insurance from my home country valid for Thailand Pass COVID coverage, or must I use a Thai insurance company?

Bobby which are the Thai insurance companies.

You can, just be aware if it will cover you for asymptomatic condition. When in Thailand if you are tested and found positive you will be sent either to ASQ or a field hospital, even if showing no symptoms. The same applies if you were sitting close to someone on the plane who does test positive. The average overseas insurance will not cover you for this, and you will be liable for tens of thousands of baht costs. The Thai covid insurance will cover this, so no financial loss. For myself, I would not take the risk, just for the sake of saving a few dollars on the premium. But it's each to their own of course!

Yes you can.

They usually require a cover letter specifying that your insurance covers covid