First, I’m not an expert, but have been running Zwave for a while. Like you I did start with 2, but located the pi and z-stick about 3 feet away. A good reference of Zwave routing Z Wave Routing Basics: Retry Strategies
Also there is an application Simplicity Studio that will measure z-wave network health. it is free, but you have to register.
Lastly there is a web site to clean you debug file. Z-Wave Log Viewer
I ran yours through it.
good luck