Make the items totally unbreakable with the same material as it is made is a bit over-powered. You should, when an item is renforced, add to it a second life bar. So when you reinforce it, it has 2 times more life, but not totally unbreakable. And add 1 item, the rarest you can, that can make any item really unbreakable. It would be better for the gameplay. Else, nothing to tell you, nice mod, great idea.
I like the idea of reinforcing it to increase it's durability instead of just instant unbreakable. (In fact, even I don't use this mod, because of the instantly unbreakable)
The mod currently also isn't true unbreakable, it will eventually break, in about 65 some thousand uses. (uses should be set to 0 to actually make it unbreakable, not sure if I did that or not, been a while since I've looked at this mod)
Instead of changing this mod, I might instead make another one with this upgradeable concept. (Possibly depreciate this mod as I think I can make a better mod that's more balanced)
Maybe also add a very minor speed increase too?
I think Diamonds are rare... so maybe every X upgrades will require some of them.