What should I do if I missed my 90-day report in Thailand before leaving the country?

Don't do anything. If they pick it up next time you have an interaction with your local immigration then pay the fine then.

C

Nothing would happen upon your departure from Thailand when you leave Thailand without doing 90 days report within the due date. No way to pay the fine at the airport.

What would happen if you just leave is, the immigration officer may or may not find out you missed 90 days report when your next application for extension, 90 days report in person, or any other thing at the immigration office. You will be fined when they find it out.

So it's your decision whether you want to do 90 days on Monday and clear the fine, or wait until next time to go to the immigration office.

The fine of 2000 thb stays the same if you do it tomorrow or months later.