I'm sort of expecting the argument here that "as anyone can use them this means that the class isn't powerful or versatile at all, it's the items they're using". But I'll wait for such a claim to be made before I get into a discussion of that nature.
I was going in that direction, but not exactly. I was going to suggest instead that the reduced gold cost be removed, instead giving a little bit of "virtual gold", ala the artificier's Crafting points, that can be used to make alchemetical items per day. Combo that with a reduced number of inventions, and the Engineer can use inventions for big issues and whip up little items for smaller problems. My issue with your version is that it has a inventions known, which I dislike because I like that the engineer can adjust from day to day. I'll let you address that before I put up more stuff.