I have constantly searched for a z-wave door knob to replace it but there isn't one.
I too prefer the door knob itself to be locked rather than a separate deadlock that could either not align & engage, or potentially lock while the door is still open.
I had a proprietary one on my holiday home - a LockState 6i. VERY solid construction, loved it, but entirely dependant on talking to their mothership. Many, many, many bugs later I have eventually removed it.
But only after finally finding a single Z-Wave lock in Australia that is also door knob/lock - it's called a KAS lock. Installed that and it is working an absolute treat - just doesn't feel as solid as the old LockState, keypad isn't as nice or robust, and it plays stupid tones you'd expect from a cheap Chinese device (which is ultimately what it is).
More importantly, both the KAS and the old LockState still had a manual key - though with the KAS it is concealed so not obvious it is there.
My ultimate goal is to tear apart the old LockState and convert it into Z-wave using a Z-UNO so I can put that baby back. If that works out I'll do it again for my own home.