Can I switch from a non-B visa to a marriage visa after my work permit has expired in Thailand?

You would get documentation from your employer go cancel your work permit then go to the immigration office and cancel your current extension of stay with termination paperwork from your employer AND then immediately apply for a new yearly extension based on marriage to a thai by showing you meet the proof of funds (400K baht in a thai bank account in your name only for 2 months before you apply) and turning in the required documents, photos, copies, bank letters, etc.

GO to your immigration office with your thai wife and and get the hand out listing the requirements to get a marriage extension.

You're NOT getting any new VISA 😮 all you are doing is changing the reason for your extension from one based on employment to one based on marriage to a thai. People do it all the time