Best option to invest 700k into rentals

[David Lant]

Looking for advice.
I have flipped around 20 properties and have previously owned four rental properties so I have a good amount of experience with the rental world.
But I need wisdom and advice. I've always believed in buying properties outright with cash, but this scenario I see will not get me to own a lot of rentals fast.
I have 700k in cash to buy rentals. What I'm thinking of doing is using the money to purchase a bunch of properties and work to pay off the mortgages as fast as possible? With this scenario, I could get seven buildings worth 300k each and put 90k down on each unit and pump all of the money left over after paying costs back into paying the mortgages off as fast as possible. They all have around a 10% cap rate.

Is it better to use the money left over after paying costs to purchase more properties and build up a portfolio that way? One issue I have is that I'm 46 years of age and don't want to be in a situation where everything is paid off and one foot in the grave.
I would love to hear people's thoughts and opinions on what is the best route to go.
Thanks.