A couple observations:

• Sewing needs some kind of quality buff. The percentage condition boost you get from repairing is almost always partly wasted, and higher levels only increase the wastage.
• The amount of effort needed to get to level 5 mending merits a much better reward. In Interloper, you aren’t ever going to be leaving your gear to 50% condition before repairing. I don’t know what such a buff would look like. Maybe you use less cloth when you hit level 5, or maybe you can repair your gear to 120% instead of 100%.
• Sports socks are gold if you’re just aiming for level 5. 0.10kg, and very fast repair time. Hoard them, haul them with you until you have level 5. You can consume a lot of cloth fast.