Hmm unfortunately you've picked something that's a bit difficult to do, the Z5 do sound pretty unique. Downside is not many other headphones would sound quite like it.
The closest headphone I've tried to the Z5 would be the Sennheiser HD650 (demoed multiple times) and the Shure SRH1540 (own this headphone). With the 1540, there's perceptually a bit more mid-bass, and there's also a ringing at the upper mids, but otherwise it sounds rather similar to the Z5. The SRH1540 also can sound a little congested, while the Z5, for an IEM, doesn't really have that same problem. The HD650 sounds better than the SRH1540 in my opinion.
Like
@Wiz33
suggested, the new Sony Z7 might do the trick, though like you I didn't like the original Z7 at all.
Have not tried the MrSpeakers headphones but I think there's hope there. I would've assumed the open backs would sound more similar to the Z5 though, since they tend to be warmer.
And if you're considering planar, perhaps the HE560 or even the HE500 might work as well?