1. It would be awesome if the datapack recipe could copy nbt of input items to the output, this will allow us to use your mod to create and upgrade sophisticated backpacks.
2. And the items stay inside the sewing station (at the very least the needle) when you exit the gui

It would be very much appreciated if you could incorporate these features thanks.

If you want the items to stay, you can use the Storing Sewing Table instead of the plain one.

As for copying NBT, I implemented that on the ToolBelt mod but I will consider moving the logic to SewingKit and making it more generic. I can't promise when I will do it but it may look a bit like:

        "materials": [
          {
            "count": 1,
            "ingredient": { ... },
            "copy_nbt": true
          },
          {
            "count": 1,
            "ingredient": { ... },
            "copy_nbt": [ "Damage", "some value", "another/value" ]
          },