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.
Tire liners have an economy/cost upside because they're indefinitely reusable and don't cost all that much to start with, and the thick commonly seen ones (Stuffy, Rhinodillo, others) do they do their job very well. They block common intruders like glass, road debris, and thorns very effectively. They have the downside of a rolling resistance hit, making you a little less efficient on the bike.
Many riders just don't care about or notice that downside, and have an easy time riding and forgetting. Shorter distance rides in an urban environment tend to push one in that direction. Liners can make a lot of sense for that rider. Others are more bothered by it, and lighter armored tires become more attractive.