Summoning items! I'm glad it happened.
- Dagger Bond: Wording seems okay. The damage die increase is a nice perk. The bonus action cost is perhaps a bit much.
- Knife at the Ready: As written, any knife you throw loses its magical properties and disappears, even if you don't choose to draw a duplicate of it. It's clear by context what the intent is, but you might want to change that last sentence to be specific.
- Hidden Cuts: Does Hidden Cuts also grant you the supernatural ability to know when someone has noticed you? Without that knowledge, you can't know to use the Reaction, but with it, you get a pretty significant perk for infiltration.
- Primordial Blades: Half of these are duplicating cantrip riders, so I'll talk about the other half. Acid and Poison are fine as-is. The poisoned condition is one of the weaker conditions and lots of stuff is immune to it besides. Fire's effect does have a large damage potential but its caveat means that a proactive team of enemies could even take 0 damage from many strikes. Thunder is the issue for me. Since you aren't locked into an element until you use the action, there is no opportunity cost to having thunder available. And since it performs so strongly in its small situation, I think it needs to be toned down.
- Symphony of Daggers: A fun capstone. As it's basically a 5th level spell 1/long rest it isn't out of line. The only thing it adds is that you can sneak attack with the animated objects and they count as +1 weapons. The former mostly compensates for not being able to use Cunning Action effectively during the Symphony and the latter just make this a little bit more than a simple 5th level spell.
Overall I think you're great for theme and delivery. Just a couple of places to tighten up wording or give intent clarifications, and Primordial Thunder needs to be tweaked.