6.75% mortgage. Pay off early or invest?

Trying to come up with the optimal financial solution. I got a 400k loan less than a year ago and I have an extra 3k/month plus annual bonuses that can be anywhere from 0 to 60k. My goal is to pay off the house as possible, ideally in less than 10 years.

Should I:

• Make extra payments to principal every month for a guaranteed 6.75% return

• Invest into an index fund like SP500 for 8-10 years until I have enough to pay it in full. Refinance if the interest rates come down.

• Park it into a 4% HYSA for liquidity and balancing the risk