Not a good idea. This is something that looks good in theory, rarely works so well in practice. Let me offer some reasons. Paying off credit card with mortgage is a bad idea all around. Yes, interest rate is less, and it is tax deductible - but now you are repaying over 15 or 30 years. With decent budget discipline, you can payoff that card in a few years using cash flow. Furthermore, most people who payoff credit card debt with mortgage money go straight back to using the credit card and running up debt again. Same argument for buying a car. Never use 15 or 30 year loan to buy an asset you will only keep for 5 or 10 years. And if something happens and you can’t make payments for some reason, you put your home at risk. Why do that?
As for borrowing to invest, that is called leverage. It magnifies your risk many fold. If you borrow 100,000 and invest, and we have a financial crisis where your investments lose 50% - your asset is now only worth 50k while your debt is still $100k.
Be proud of the fact that you have paid off your home. Payoff your debt with cash flow, and leave the financial engineering to the folks at Goldman Sachs.