you can apply for the extension in Thailand at your local IO. To apply for another 90 day non - o, you have to apply from outside Thailand and get a new entry stamp
Best to apply for a 90 day non 0 visa from your embassy. That gives you the time to open an account in one month and hold the 800k funds to meet the requirements for a 12 month extension. Good luck
Traveling overland by train, bus, etc to Cambodia, Laos, Malaysia (including en route to Singapore), Myanmar, etc is not accepted as proof of exiting Thailand.