The single biggest improvement available to you is timing, not technique: mend it before it opens.
A thin patch with the weave intact still has something to stitch into. Once it has actually gone, the surrounding fabric is already weak, so you are anchoring a repair to material that is on its way out, and that is why second repairs so often fail next to the first.
Catching it early also means the repair is smaller, flatter and genuinely invisible. Run a hand down the inner thigh of the pairs you are still wearing — you will feel the difference between denim and nearly-not-denim immediately, and that is the moment to spend twenty minutes.
A couple of things that slow the wear itself, which is worth knowing since you say every pair goes the same way:
- Wash cold and inside out, and skip the tumble dryer. Heat and tumbling are hard on already-thin fibres and this spot is always the thinnest.
- Rotate pairs rather than wearing one until it dies. Fibres recover somewhat between wears.
- Fit matters more than fabric. A very close fit through the thigh wears there faster no matter what the denim is, which is why it is always the same two spots on you regardless of which pair.