I know cheap inner tubes can be a bother this way, they sometimes tear for no good reason. At the bike shop they have a couple of times thrown out entire boxes of bad batches of tubes, I can't remember the brand name at the moment. Try a more trusted brand next time, at least a different one. It's common to make tears in the tube when you put the tire back on, be very careful when using tyre levers. In most cases you can get the tyre back on with otu levers by pressing the walls of the tire between you fingers and making sure it settles well into the rim all round the wheel, then the last bit usually snaps into place with ease. Pump some air into a brand new tube and let it out again, it stretches it and its easier to get in place under the tyre. Most tyres with kevlar guard are acceptable, but the best this way is Schwalbe's Marathon line, I have had Marathon Pluss (optimal puncture resistance) and Marathon Green Guard. These are havier tyres, there are fimer and lighter ones like Delta Cruiser with kevlar guide.

Once I kept on getting short slits in my tubes, couldn't find anything wrong with the tire, replacing the tire and tube made no difference. It turned out that the tire liner had cracked and was pinching the tube. I had to inspect it by bending it out.

I've also picked up the tiny bits of wire others have, couldn't find them with bare fingers, cotton turned them up.

Public service announcement: June 1st is National Doughnut Day in the U.S. You Canadians can go have some poutine... or ice cream... whatever suits your fancy.

https://en.wikipedia.org/wiki/National_Doughnut_Day

It is often easier to find the culprit if you start with the rim, tire, and tube all together.

Mark valve on tire, pull off tire and tube. Then locate the hole in the tube. Determine if the hole is tire or rim side, then inspect the tire or rim near where the hole was.

Sometimes one can dig out a piece of glass, only to discover that the actual cause of the flat was on the other side of the tire.

Pinch flats are sometimes secondary to punctures, and can be deceiving.

That tube was the stock one that came with the bike.

I agree and now have two Schwalbé Marathons supremes.
They are heavier as you say but the rolling resistance is even better thwn on the marathon plus.

There is a Tim Horton's Six bike pedals from my house so every day is National Donut Day. :)