As far as the procedure is concerned, you have received very good information. However, I would like to emphasize that long-term stay without insurance is very negligent.
I've been to the Immigration Chonburi (Jomtien) last week for information. An officer told me, that I can come 40 days or less before my stamp expires. I suppose they change it because of Songkran holidays.
I must add, that the officer at the Immigration was very friendly and helpfull, may-be because of my age and especially when she saw, that I was many years in Thailand with a Retirement Visa I always had before Corona.
Live in BKK please contact Bangkok immigration. Live in upcountry please contact immigration where you live." TM87 is the form for the Change of Visa Exempt to Non O-IMM and Form TM86 is for those who want to change their tourist Visa.