Can I renew my multiple entry NON O marriage visa before the one year period is over?

You can apply for a new visa when it's expired at the consulate outside Thailand as many times as you want. Usually, consulates wouldn't issue a new visa when you have a valid visa.

Or can apply for a 1-year extension at the immigration office inside Thailand at the end of your permitted-to-stay stamp.

Remember, for those 1-year multiple entry Non-immgratnt types of visas, you can stay in Thailand for up to 15 months total. The "valid until/must be used by" date on your visa is the last date you can enter Thailand on that visa and you will be stamped for a new 90 days.