Can I extend my TR visa by 30 days and qualify for a COVID extension in Thailand?

You never get 4 Month on TR Visa.

At MOST immigration offices you can apply for an extension on your entry stamp when you have 30 days or less left on it (Jomtien is 10 days or less)

BUT

Some immigration offices make you apply for the regular 30 day extension FIRST before you can apply for a covid extension. AND almost all immigration offices that allow you to apply for the 60 day covid extension start it on the day you apply (they do no add it to the expiration date you have now).

GO to your immigration office and see what you can do

One last thing, the very last day you can apply for a 60 day covid extension is the 30th of March NOT the 31st :P