Apply online for the Non O visa for over 50s. London Embassy is easy to deal with. This gives you 3 months then you can get 12-month Extensions of Stay each year.
You’ll need a Thai bank account with 800k baht. No insurance needed for each Extension but you will need insurance for 3 months for the initial visa.
If you have a problem it will be from the airline at check in. There’s no way to be sure how strict they’ll be. Some insist on a flight out of Thailand within the 60 days, others will say nothing.
You could book a cheap flight out beforehand, you could buy an onward ticket service from one of the companies that provide this or you can wait until check in and be prepared to buy a ticket on your phone if they insist.
eg tell them you are going to visit Cambodia or somewhere but not sure exactly one and then book the ticket if they won’t check you in without it.
My only concern is if the agent gets your extension from Bangkok and not from the province where you live. If they get it from your ‘home’ Immigration Office then that’s fine, if not it would just niggle at me a little.
you can’t use the income method for the first year because you won’t have 12 month history of transfer from overseas each month. After that it’s one method or the other.