I always do the extension in the last days. 2-3 days before the end is enough time to get all paperwork done. But lookout for Thai holidays - I have added the Thai holidays calendar to the calendar app for that. (Important notice: if a Thai holiday falls onto a weekend, the next workday is holiday!)
Immediately open bank account and send 800k from abroad.
After 80 days Goto immigration and ask what exactly they want to see. (Bank statement and certificate, a passport foto, book bank with a transaction of the same day, ID of Thai wife, your passport,…).
Get those things and return with 1900 baht.
If you plan to exit more than 3 times per year, get a multi reentry permit.
I just did my EOS yesterday as I’ve been doing it for over 10 years now. Costs 1900 baht and 5-20 minutes waiting (and time doesn’t count anyway for a retiree ;) )