even a family business must be registered with the authorities, show them that along with a letter from one of the partners giving you permission to work remotely from Thailand
that is absolutely not true. You can only work remotely on 2 specific visas. A DTV issued for workation and the LTR Remote Worker visa, why do you think these visas were introduced?
If you want to work remotely then you need to apply using the workation option. The soft power options don't give permission to work remotely. You can sign up to a gym independently