Work Permit (sole practitioner)

I’m fed up with being retired and I want to create a small single practitioner practice in my field (psychotherapy/talking therapy). I do not want to work for someone else (eg a hospital), and I want to deal only with English-speaking people (including Thais who can speak English well). I am thinking of changing my visa from a retirement visa to one based on marriage to a Thai (several years, I’ve lived here for 14 years and in my present house for 12 years).
Will changing to a marriage visa benefit me? What is the best way of legally establishing a sole pratitioner consultancy? Anyone know?