Can I leave Thailand after getting a non-O visa extension, or do I need to stay?

You could indeed come in on a 90-day single entry non-immigrant type O Visa, open a bank account, put in the 800 k, then buy a re-entry permit for 1,000 baht at the immigration office where you live/stay or at the airport before you fly out, exit the country and as long as you come back before the 90-day stamp runs out you can apply for the year extension.