I think it's nice. I don't think it's unrealistically long (4 syllable names are common enough), and it's not difficult to pronounce, so I'd disagree about it being too much.
The vibe I get from it is sort of like a cross between
Zoe and
Octavia. Spunky, stately, a little new-agey (I think because of the possibility of Zen as a NN).
I don't love Zennie, but it'd work. Reminds me of
Lenny.