Jeans wear through at the inner thigh long before anything else — is there a repair that survives more than a few washes?

The repair that lasts is a patch on the inside with dense machine stitching over the whole area, and the two things you tried are the two that reliably fail here.

Why this spot is different from a knee: it is not abrasion against the outside world, it is the two sides rubbing against each other, constantly, with movement in every direction. So anything sitting proud on the outside gets attacked immediately, and anything relying on adhesive gets flexed and heated and washed until it lets go. Iron-on patches are for low-movement, low-friction places, and this is the highest-movement spot on the garment.

The method:

Cut a patch generously larger than the thin area, not just the hole. This is the part people get wrong — the fabric around the hole is already thin and will go next, so a patch that covers only the hole moves the problem two centimetres. Feel for where the denim stops being soft and go beyond that.

Pin it inside, round the corners so there are no points to lift.

Then stitch back and forth across the whole patch in close parallel rows, on a machine, with the feed going in one direction. Rows a few millimetres apart. You are not attaching a patch, you are creating a new dense fabric across the whole area — that is what makes it flexible and flat instead of a lump with a hard edge.

A matching thread makes it nearly invisible from the outside. A contrasting one looks deliberate. Both survive; it is only a decision about how you want it to look.

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.