@maggiehanley I suggest reading the wikis on r/personalfinance and learning more about it before commiting anything. It's not complex and will help greatly. It will also spark other questions you can ask that are related to your understanding and situation.
In general it does not make any financial sense to pay extra on a low mortgate like yours. You can use the money much more effectively while also keeping some liquidity if needed. I personally recommend not making any major decisions until you learn a bit more and settle into your new lifestyle. There is no rush.
Your question of highest return is nuanced as at that point it depends on your other finanical aspects like age, lifestyle, debt, risk aversion, life desires, retirement, kid expenses, college, health care, etc.