Dilek

Dense stitching over the whole patch rather than around its edge. Edge-only is why the first attempt always fails.

On when a pair is not worth saving, since that was your second question and nobody enjoys deciding it.

My line is whether the surrounding fabric can still hold stitching. Pinch the denim a few centimetres either side of the worn area. If it feels like fabric, patch it and it will last years. If it feels papery and you can see through it against a window across a wide area, the repair will hold and the fabric next to it will open within months, and then you are patching a patch.

The other genuine end point is when both sides have gone and met in the middle, so the repair spans the whole crotch. That can be done, it holds, and it is a noticeably thicker area to sit in. Worth it on jeans you love, not worth it on ordinary ones.

One more thing, since you mentioned the stitched-on patch feeling like a lump: if you do not have a machine, the hand version of the same idea is darning: rows of running stitch back and forth across the thin area, then the same at right angles, worked into the good fabric all around. It is slower and it lies just as flat. What makes a repair a lump is the hard edge of a patch sitting on the surface, not the hand versus machine question.