There is no such thing as pre retirement visa. But your plan will work. After covid is over. UK or no UK I do not think it makes much difference unless you are coming directly from the Space Station.
What kind of people live here for years on these visa runs? I thought visa runs were over long time ago, but they are still around. What are they doing here? Selling drugs?
Frankly I signed for this group to have more info about covid. As soon as covid is over, I will leave immediately. Expat community is full of myths and false information. Filter, use your head and research yourself. Learn Thai.
Do not listen to the visa runners. Your visa will be extended for the duration of your contract without you leaving. If it is a good school, they will do almost everything for you. At this point all schools are closed. If they start function on time, your visa will be fine. Find someone who teaches there, and you will know specifics of that school.
For several decades I hear this song again and again "they need us to spend our money here. We all leave and TH will be in trouble". It never happened.
I do not believe this clearance is relative to non-crimes in your home country. Imagine a guy from Nigeria or Kuwait or Papua New Guinea. Are they going to check the record in his/her country? For 100B? Impossible.