Should I buy average tires with puncture resistant tire liners or just puncture resistant tires? The reason I am asking this is I am a pretty big rider who carries cargo sometimes. Let us say close to three hundred pounds all told. The streets where I live are terrible with many pot holes. There are also a lot of construction sites that will be there for the next few years. I got nine
flats in one month/150 kilometers. They are not pinch flats. All are at
the exact center of the outside of the tube. They are not always in the same spot either. Only once have I seen any debris embedded in the tire and I didn't think it was responsible for the flat. They are the stock tires from when I bought the bike a 2017 Norco Yorkville. I would just bite the bullet and buy great tires like schwalbes but could really use the funds elsewhere on the bike. I forgot to mention. The tires are inflated to 75 P.S.I. close to the 80 max.
Exact center of the outside of the tube (and no debris) indicates a pinch flat.  Why do you assert not a pinch flat?

My experience with tire liners is they abrade the tube.  I would wear out a tube a year riding about 500 miles.  

Puncture resistance tires work better for me.

Why you are not seeing debris in the tire is a mystery to me.

You might also try tubes with sealant for small punctures.