How soon can I apply for a 12-month retirement extension in Thailand before it expires?

Suggest you go to Jomtien Immigration and ask, with reasons why you need to obtain the one year extension earlier than 30 days. I know that Jomtien has granted an extension earlier as an expat friend did it earlier. He had his Thai wife submit PROOF of his departure airline ticket and the need for him to return to the USA for an important medical review date with the US Veterans Administration doctors. His request was granted.

There were a few reports someone successfully negotiated and managed to apply for a 1-year extension in advance, but I don't think those from Jomtien.

You go to the I/O with the proof that you are NOT in Thailand within the application window, then ask them if they accept your application before you leave. As proof of that, you need an international round-trip ticket with a fixed date on it. In your case, you may try with the tickets leaving Thailand someday in March and arriving in Thailand after 2nd May. Open tickets wouldn't do a trick.

Don't know if this works at Jomtien or for you. You can try it if you want. The worst thing is they say you cannot. In that case, you wait until 2nd (actually 3rd as it's Monday) of April or do the whole process again.

Unless I'm mistaken Jomtien allows you to apply when you have 30 days or less left on your current stamp

So as long as you meet the requirements you can apply any time after April 2nd.