Just a report from Chiang Rai Immigration.
2nd Covid extension until 28 Feb 2021 was no big deal. No embassy letter was needed, only the usual paperwork and a copy of the rental contract.
Immigration officer didn't know about any possibilities beyond deadline of 29th January and advised to come before this date for checking new rules. He couldn't answer, whether it will be possible to go for a 3rd extension by end of January.