Which dagger should I 4*+10? (Also I want to give the unit the kagami mochi)

Wrath is restricted to melee infantry and armored units, unfortunately. Maybe something like Desperation?
I would recommend Kaze, since Kagami Mochi and Barb Shuriken are literally the same thing.

Well, I don't know if Astra would be that useful on him, but I'd recommend Kaze as an easy merge dagger unit. And even though Barb Shuriken+ and Kagami Mochi are like the same thing, why not?

One very important thing: Daggers (or any ranged unit for that matter) can not inherit wrath.

Additionally, I would highly advise against inheriting Kagami Mochi to someone as NY Takumi has valuable fodder in Atk/Res Bond, and Kaze is available in the 3-4 Star pool to inherit Barb Shuriken (same effect as Kagami Mochi, though you'll need feathers, of course).

That being said, because you want to build a 4 star +10 dagger with flashing blade for the astra procs, you'll need fast daggers in the 3-4 Star pool. In my opinion, Gaius or Kaze are your best options (Felicia outspeeds them, but has less attack and would probably prefer her own weapon anyway). Kaze is probably optimal since he comes with Barb Shuriken and has 1 more attack than Gaius.

If the lack of wrath is concerning relative to proccing astra, perhaps you can make use of Infantry Pulse, though this only works on the first turn, of course.

Good Luck!

Kaze or Sothe

Kaze if pretty much the de facto 3-4* dagger unit for merging rn, unless you want to run some weird tanky debuff builds I've seen on some Saizo/Mathew's that utalize Smoke/Rouge Dagger.

And as mentioned before, Kaze's barb shuriken is litteraly the same as Kagami Mochi.

Astra seens kinda overkill though IMO. Can't really think of many units that has enough HP to justify running Astra over glimmer/bonfire/glacies, and if your dagger unit run up against tanks Luna/Moonbow would be much better.

LEGAULTTTT

Originally I was going to suggest someone fast who can use desperation moonbow well, but the examples with enough speed for my satisfaction, but generally I'm finding that daggers have too low of attack for my liking for such a setup.

Do you have HS Xander? Kagami mochi plus ignis is a party.