Is it advisable to cancel my Non-B visa and apply for a 30-day extension before my next Non-B visa sponsorship?

Did your new employer tell you what 30 day extension to apply for? Coming off of a non-B you generally don't qualify for any extension and need to leave the country the day your existing extension is cancelled.

You MIGHT be able to apply for the Covid extension, but this is not normally an option so your employer gave you terrible advice. And they need to check with the immigration office they plan to use for your new non-B extension and find out if you can get that extension if you're on a covid extension.

But all of this is assuming you are even approved for a Covid extension which is not guaranteed.