that's new information. For decades all kinds of work has been illegal on a tourist visa, including online work towards foreign clients and even gardening in your own house.
But this latest government did seem to try to modernize the country before all the politicians started squabbling like children again.
Nothing. No fine. The immigration at the ports don't care about the 90 day report. It's only the in-country immigration offices that use and enforce them.
I flew out on the 3rd, having stayed 148 days, without doing a report.
The best solution would be if you took a 2-3 day trip to Malaysia or someplace similar. That way you get 120 days total in Thailand and you get to visit another country.