Ok, so that’s not the issue. Hmmm….
At this point, thinking as a field tech, I would be wondering either about a flawed SmartThings Z wave implementation, or some kind of local interference.
My own next step (and I realize this might be way too much work) Would be to get a Z wave hub of a different platform, probably the Zooz z-box just because it’s probably got the least friction in setting up, and try adding the problematic devices to that. If they add smooth and easy, The problem is either smartthings or an individual bad device still connected to the old network, which is spamming everything.
If they don’t add to the new platform either, it’s a local environmental issue. Baby monitor or drones or something. Maybe something on a pulse frequency that just happens to disrupt the exchange of encryption keys for the newer zwave devices. Or, you know, poltergeists.
(an argument against the Z-box is that it won’t work as a secondary, so you might prefer doing this test with something like hubitat. Choice is good.)
So, let’s say they do add to the new network, smooth and easy. The next option to consider is what you were already thinking about doing: removing one device at a time from the old network to try and see if there’s one that’s gone bad.
Or you could go ahead and use real diagnostic tools like the zwave Toolbox. It typically costs $130 on sale and it’s pretty easy to use, and it can save you tons of time, but then you’re getting really deep into the technical stuff and I don’t know if that’s any interest or not.
Or, if the hub from a different brand that you got for the previous step can be added as a secondary, you could try adding it to your SmartThings set up and then using the new hub’s diagnostic tools. Instead of getting the toolbox. That could have the advantage of being a newer Z wave generation, so better able to diagnose possible issues with newer devices. Just fewer tools to use for the diagnosis, but Almost certainly way more than smartthings provides.
So it really comes down to you how much time and effort you want to spend on diagnosing the issue, or if you just want to give up on zwave with SmartThings. (And hope a similar problem doesn’t arise in the future with Zigbee or matter.)
There are some community members like @johnconstantelo Who have over time moved pretty much. Their whole smartthings setup to Zigbee and been pretty happy with that. But of course, no guarantees.