Will I overstay my Non B Visa if I leave Thailand on May 2 after it expires on April 30?

30 April is a Sunday so you can't do anything that day. I'd suggest you get documentation from your employer and cancel both work permit and extension of stay in advance. Then it is up to you whether you risk overstay or leave on the 30th but the latter would be more sensible

A non-B is obtained based off a work permit. As soon as you stop working or your work permit expires, your non-B is no longer valid, whether it's cancelled or expired.

So if you are still working until April 30th then at that time you'll need to cancel your work permit and your non-B visa extension and you'll have until the end of that day to leave Thailand. You cannot extend it unless you are working and have the support of your workplace.

You'll likely need to plan to leave a few days earlier unless you're willing to risk overstay.