Should I cancel my Non-O visa before going to Laos to apply for a Non-B visa?

IF you have the stamp Extension of Stay based on being a dependent, cancel it one or two weeks before travel. Do not wait until the day of departure to run into an Immigration Office and when done hope that you still make it on time to the airport.

Make sure by reading the website of the to visit Consulate that you have all documents with you to apply for the Non Immigrant B visa.

Good luck.

Are you still on initial 90 days from a Non-O visa based on being dependent on a foreigner who stays in Thailand legally?

If so, you can just leave. The initial visa cannot be canceled anyway.

But if you have already applied for a 1-year extension once or more from a Non-O visa, that means you are on an extension and not a visa, you'd better cancel your extension. Or you might or might not get a problem when you apply for a 1-year extension after entering on newly obtained Non-B.