You can extend in Chiang Mai. You will need to show all original qualifying documentation plus residence details. The 500k has to be in the bank 30 days before the application - it may have to be in a local bank but please double check.
I think the path of lease resistance is to do a same day border run.
You can complete the 90-day report 14 days before and 7 days after the 90-day date. The TM47 form you need will be available at the office and takes about 2 minutes to complete.
For the extension in country immigration are asking for an updated letter. It seems to be the letter staying you are an employee and allowed to work remotely in Thailand. This is NOT being asked for in re-entry from trips abroad. There have been a couple of people posted about this recently. One in Krabi and one in Phuket