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.
You can’t use it for the first year only subsequent years. You’ll need to be able to transfer 800k into your Thai bank account immediately when you open it for the first year, or pay extra to the agent if you use one - but that ‘might’ complicate it for the second year.