Just a report about process at Chiang Rai Immigration: :
ED-Visa would have been expired 28th October. IO said that there is no possibility to extend the ED Visa within the kingdom.
So ED Visa was cancelled and new application was for 60 days COVID extension.
Immigration asked for copies of rental contract and passport only.
Got stamped in ' under consideration' until 11th November. Officer confirmed that extension will be 60 days.
Costs: the usual THB 1'900,-.
All quite straightforward and surprisingly simple compared to ED-Visa extensions with tons of paperwork.