If you qualify for the LTR then great otherwise you have 2 real opions, either an O or an OA visa.
The OA is multi entry & is applied for in the uk, needs health insurance & proof of funds in uk account along with police check & doctors cert. All the information is on the Thai embassy website & it's not as complicated as it 1st seems. To extend you will need Thai bank deposit or 12 monthly deposits & health insurance.
The O visa can be obtained in the Uk or in thailand valid for 90 days. You would then go to your nearest Immigration office to get a years extension based on retirement at which point you need to show 800k in a Thai account which has been seasoned for
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months. You don't have to have health Insurance but it's still a good idea. Both of these can be extended 12 months at a time & it is possible to have a multi reentry permit or to buy a single re entry permit.
You can use the monthly income route but only on your second year extension as you must show a deposit of 65k every month for the previous year.
You are not allowed to work here on a retirement visa but if you're really just answering emails then it shouldn't be an issue.
Hope this helps