Not to brag, but I think I've got one!
Nyx Eudoxia - I mean,
c'mon, it's got the echoed x without sounding too repetitive, and the flow works well for me. ;o) Really, what's not to love?
I also like
Nyx Meritxell, which has the double-x look, but not the repeated x sound. It's nice, but I don't love it like
Nyx Eudoxia. Plus, the meanings are opposite of each other: "night" v. "midday." :oS
There's also
Nyx Ruxandra, which IMO, is a winner also.
You're right; it's harder than it looks!
My brain already hurts, lol.
I love all the combos you came up with
Nyx as the MN, and of the combos w/Nyx as the FN, I absolutely adore
Nyx Elektra.