In the end it's easier to border bounce. Extending is a real hassle you have to go to immigration you have to provide all the updated paperwork from your initial application plus you have to show the $500,000 in a bank for a minimum of 30 days then you have to wait a 2 week under consideration period while they send the paperwork to the main office.
You can't have any business interactions with Thai entities, clients, etc. the business needs to be registered outside of Thailand. Although there is no income requirements you'll need to show a viable real business with contracts and clients or a history as a freelancer.
You'll need the bill of sale, copy of their Thai ID and house book, you'll need the transfer papers completed, you'll need possibly two power of attorneys signed by them so that you can change the bike plates and register your name in the green book to your province
Seems like getting a residence certificate is becoming more of a challenge since my last experience maybe 6 years ago. My recent experience in Chiang Rai was similar, filed TM30, recent entry stamp and visa, landlords house book and ID, and house book of my rental property. No fee asked for, wait a week.
A visa only shows the purpose of your visit. If the immigration officer isn't satisfied with your answers regarding the reason for your visit you can be denied entry.