Fyi: went to Chiang Mai Immigration for 60-day Covid extension. I got my 'under consideration'-stamp 17 Feb (2 weeks before my TR visa finished). The 60-day period starts counting at the day i applied, so until 18 April.
So maybe better go by the end of your Visa-validity to profit fully from the 60-day extension.
By the way, with my under consideration stamp i got the new stamp within 10 minutes.