They look good
I can see your learning curve.
What I've used on my cards is a pre-creased cardstock that is sold flat in packs of 20 cards I believe. The card stock is also textured, which I found looked better than the flat matte paper because it at least gave it some texture then. I printed a few on the plain matte because the store ran out of the textured paper and I was a few Christmas cards short
I also learned that if you make the picture a little larger so it goes just over the fold and slightly on to the back it actually works a little better (though this advice might not work because your cards are scored instead of folded).
Have you tried looking for paper that's matte on one side and glossy on the other? It appears that Ilford may make some like that. At least then you should be able to write on the inside of the card in normal pen. And if it were plain paper you could just score the fold yourself using a plastic tool I would think, but that introduces a new set of steps to both cut and fold the cards yourself, which may not be desirable if your great-niece is making a lot of cards.