I had a similar situation to you last December, my B400k would be 2 months in the bank on the same day my extension (non O Thai wife) was due. I went in to the office weeks in advance to talk to them. They told me that I could not get the extn until the following day which would make me one day everstay. The IO advised that I could get a short term extn - visit family - or go OS. His advice was OS 1 day and keep the family visit for any emergency in the future. He said I could only get one per visa.
This was in the Kanchanaburi office, probably different wherever you are, so go in and have a chat
I am in a similar situation to you. I had a ME Non O (Thai wife) visa issued in Suvanalhet. In December last year I extended for 1 year, in country Non O, so no more border bouncing.
Today I went to immigration to ask about changing to retirement and they said as long as I had 12 months of B65k per month deposits from abroad to my Thai bank account, no problem.
Britain had a dual tax agreement with Thailand, so you won't be taxed twice on your pension. I am also on a UK pension and I enquired about what to do, if anything, and that still appears to be a grey area.
Unfortunately there are a few paid up members of the "you didn't want to do that" club who just have to be critical. It must be great to be perfect all the time and never make a mistake!