Use Google Flights and set up an alert to send you an email when prices are down. I have mine set up to fly to the US from Phuket and sometimes get alerts for around 25,000b, around $800, I think.
What a lot of organizations do to work around the WP for Volunteers is that they call a paid experience as volunteering. They still call it volunteering which is ok, I guess, but in reality, the person is paying for whatever experience they are doing. So technically they are not "volunteers." They are paying tourists. This is really common in the elephant tourism industries here.
If you have a US address, you should keep it. Many US banks don't accept non-US addresses. And a BIG thing with having a US address with the SSA if you're getting money from them, is that you won't have to send a certificate that you're still alive every year. If you use your Thai address, you have to do this. And if for some reason they don't get it, they cut you off until they do. It could be lost in the mail which happens fairly frequently and you won't know they don't have it until you don't get your money.
Sometimes you don't have the choice. I stupidly got into an argument with the head of my immigration office and now she won't even do a 90-day for me. Thank goodness I can do it online.
Who is holding your hand in depositing money in a bank account? You just need a Thai bank account with any amount. You have to provide a lot of documents and physically go to the office at LEAST once and usually more than once. Plus you need to go to your bank and pay fees for statements. I rather just mail in the 3 things I mentioned and wait for it to come back with my extension. To me, not having to do any of that other stuff is worth paying someone to do it for me.