The S&P return is not always and certainly not guaranteed to be 10% or even positve. Some months/years it is negative. It's an exceedingly bad idea to keep cash that you will need in the short term (< 5 years) invested in equities.

If you sell OTM covered calls of about 10 or 8 delta against the shares you own and keep rolling them after sufficient theta decay you can conservatively add 1% ROI each month on top of whatever appreciation your shares get. So yea you have a good chance of getting 10% ROI per year consistently this way unless S&P is down more than 2% for the year.

Unfortunately so many people use this strategy these days that the bids are lower than what they used to be though.