I did end up changing my return ticket date to Sept. 24th.
One scenario I was thinking:
Enter Thailand on visa-on-arrival, get a 1-month extension on that = 2 months. Before that expires, take a 2-3 week trip to Vietnam. Apply for a SETV there = 2 months plus 1 month extension. Total = 5 months. I turn 50 on January 16th. Would this work? THE SETV requires a return ticket out of Thailand so I'm not sure how I'd handle that. Just get a cheap flight-out ticket to wherever?
I'm trying to avoid having to apply for a SETV or METV in the States since it is now the 18th of July, the processing time alone can take "a minimum of 15 days" and then there is the shipping back and forth delays. So I'd be pushing my luck if I need to depart on the 26th of August, at which time I might be bereft of my passport.