Have you searched the group at the top of the page? It will give you all the information from other applicants for what documents to apply with. Search “medical” and “documents” and the embassy you plan to apply from e.g. “Hanoi”.
You have to apply for DTV visa outside of Thailand. Hanoi is a good option in Vietnam. Plan to be there 2 weeks but it could be quicker / longer. For the Vietnam visa ask for 2 months on the visa form just to be safe.
You’ll probably need 6 months of medical appointments or to provide some kind of longer term treatment plan. You often don’t have to pay for it all up front, so if you can.. I would provide a long treatment plan for the visa but if you only use 5 months and go back to UK then that’s fine, as no one is checking up on you. Again search the group. Call around some clinics to see if they have had successful DTV applicants before.
You don’t need to write on here to justify your situation, or prove you know better, post in the group if you need to ask specific questions (search first, if it’s been asked before). Just get on with researching what you need to apply for the visa and focus on your health and recovery 💕