Thank you for the helpful responses, everyone, very much appreciated. Based on your responses I've devised a plan:
Visa Renewal: My current multiple-entry Non-O visa expires on August 28th. I’ll leave the country, get a 90-day entry stamp, and then extend it for 60 days, which should take me to the end of January.
Financial Preparation: I aim to have the 400,000 THB ready in my Krungsri Mee Tae Dai savings account by mid-December, or by mid-January at the latest.
Visa Obtaining: At the end of January, I’ll go to Laos to obtain a single-entry 90-day Non-O spousal visa.
Extension Application: At the end of March, I’ll apply for the visa extension with the 400,000 THB. The immigration office will give me a 30-day consideration period, which should cover the time until my current visa expires.
Final Step: Hopefully, at the end of April, I’ll visit the Samut Prakan Immigration Office and receive the one-year extension stamp.