I've worked in the bike industry as a mechanic for 35+ years. I also commute to work by bike daily 36-40 miles round trip. I fix flats every day. In my experience, goathead thorns are the most common cause. I don't really like slime because it has a tendency to clog up the valve cores and cause a slow leak out of the valve. I've used the heavier duty tires such as the schwalbe marathon and the conti gatorskins. Those both work well, but they ride like you have garden hoses for tires. I moved to Denver from Oklahoma last year (I'm very familiar with the goatheads in Oklahoma), and I was getting 1-2 flats per month at first. A lot of presta tubes these days have a removable valve core, so I started putting tubeless tire sealant in my tubes. The result after almost a year is I haven't had a single flat.
I made a video of an ongoing experiment I started about a month and a half ago. I installed some lighter weight road tires, tubes with Bontrager TLR sealant and used it on my commuter bike daily through the sketchiest sections of my commute. So far after 1200+ miles, no flats. I'll make a follow up video soon.