Your approach looks like the correct one.. Have you installed "Hubitat Z-Wave Mesh Details" community app yet? It reformats the existing info to give you a much better picture of your devices and their neighbors. Helps figure out if a device is starting to fail possibly..
Also @thebearmay's hub information driver as well.. can give you insights into the hub like thermal and cpu performance.
40 kbps is perfectly within spec so should not necessarily be an issue but I agree especially for devices closer to the hub - should be at 100 kbps but your environment certainly plays a factor.
In your Z-Wave details list do you have any ghost devices - entries in your Z-Wave details table that look weird?
Do you have any oddball proprietary RF (900 MHz range) controller type stuff (security systems, remotes etc) or old cordless phones (remember those?) that could be causing interference?
