Can I set up a business and be a shareholder on a retirement visa in Thailand?

You can only own a maximum of 49% of a Thai business, unless you're American and its set up under the amity treaty. You can't do any sort of work in or for the business. Only be a passive shareholder, if you want to be involved in any way, even as a director, you would need a different visa.

Won’t matter as you’ll most likely loose everything and have to

Go home anyway

You will need to cancel your retirement visa and apply for a Work permit

You generally cannot combine a Thai Retirement Visa (Non-Immigrant O-A) with investor activities in a business, as it is designed for retirees without work or business obligations. For individuals with investment plans or who want to run a business, the LTR Visa (Long-Term Resident Visa) is the more appropriate option, as it offers specific categories for investors and entrepreneurs with corresponding financial requirements and benefits.

Yes