At best, the Sharpe Ratio is a single hard coded straight-line preference curve, and the portfolio with the highest Sharpe Ratio is not necessarily the one which will give you the best chance of meeting your goals.
There is no such thing as a risk-free, guaranteed investment. Everything has risks.
We don’t normally recommend being on margin, but we recommend having the option in case it is needed.
Beware the turnover rate. It’s much wiser to invest in funds managed by people you trust.
The correlation between two investment is used in portfolio construction and rebalancing.