Will I be fined for not reporting my TM47 before leaving Thailand on my ED visa?

Now you don't even have to search.

It takes 1 minute online, why risk it?

If you are leaving for good, you may just forget about it.

But if you are planning to be back to Thailand in your future on any type of Non-immigrant visa, you NEED to cancel your current extension. On that occasion they would know you don't do the 90days report and you might be fined for that.

You need to report that you are 90 days inside the country, even when you leave after 104 days in the country. You will not get fined when you leave the country, you may get fined when you are back in Thailand and try to make a new report if you stay then 90 days or longer.

BTW You also need to cancel your Extension of Stay at the same Immigration Office where you got the stamp, it does not cancel itself or by eaving the country.