Everyone here gas said it already, this is thigh rubbing causing the damage. Adding additional fabric from the inside will not work, it needs to be on the outside, and I would recommend something tough like a harder synthetic fabric. Many brands of workwear come with different material there, but patching existing pants is not that hard, the issue is making it look nice and that really comes down to actually matching the fabric appearance and doing it neatly.

That said, a short term solution may be to patch it from the inside. It won’t stop it having visible wear if checked closely, but if your thighs touch then they won’t be visible without you spreading your legs apart quite far. The layer being on the inside will protect your underwear from the worn section and also make it more comfortable. Of you use a single continuous piece connecting from one thigh to the other it can cover all the seams may make it more comfortable, though that is down to preference.