Couple thoughts:

Glyphosate works, triclopyr is better for ToH (per PA Dept of Ag, Penn State Ag Extension). Tractor Supply sells "Brushtox," a highly-concentrated Triclopyr-based product.

You did what's called "hack-and-squirt" - a recommended method for treating ToH, but it's known to take several applications over a year or two to really work.

Try cutting the herbicide with a little bit of diesel - the thickened mixture will sit in the hacks, whereas unmixed herbicide can just trickle out.

September is pushing you luck at the end of the season. At the end of the growing season, the tree pulls nutrients into the roots for the dormant season. Herbicides must be applied before this starts. A good indicator that this has started is red-colored leaves. If the leaves are red, it's too late.

Depending on the size of the plant, a foliar application can work relatively quickly, but if it's too big, spraying the leaves may be unsafe as herbicide can blow back at you.

Sounds like you're on the right track, but may need one or two more applications this year. Just whatever you do, do not cut it down until you are certain the damn thing is dead.