Do I need to report to immigration when returning to Thailand on a multiple entry retirement visa?

Whether you file a TM30 (notification of foreigner in residence) form when you come back into the country is dependent on your immigration office and their policy towards TM30's

Some want you to file a new TM30 when you come in from overseas

Whereas

Some say as long as you're returning to the address you live at (where you got the extension and file 90 day reports) you don't need to file again

There is no way to answer it, you're going to have to ask your immigration office whether they require new TM30 when you come back from abroad.