Do I need a letter from the consulate for a 90-day exit on a multiple entry business visa in Thailand?

Or go to your immigration and get an extension for one year.

Meet the requirements

the one year is yours.

you needed to have been ahead of the curve on this, <- meaning you should have seen this shit storm on the horizon and bounced a week ago BEFORE the borders started shutting down.

You hold a visa valid for a year with UNLIMITED entries on it where each entry gets you 90 days. There was NO need to wait for your current stamp to run down to bounce out and get a new 90 day stamp.

You are NOT gonna get anything but a 7 day extension to get out of the country in a week. There is no extension provision for multi-entry visa holders.

They will tell you to go back to your country.

Remember you CAN get OUT of thailand by air still