hi @Kevin_O_Brien , @benjamin.
There are a lot of similarities / parallels in hardware, aren’t there. A couple of things to add:-
THREE 5G Router, firmware and home networking
My Three Router is set up with ‘IP Passthrough’, and the internet connection is relayed via ethernet into my Google MESH network - we wouldn’t be able to cover our home without that. Thus, the Three 5g Router is in bridge mode, and negates double NAT. I think! The 2.4 and 5 wifi networking of the the Three 5g Router are turned off.
ARC is listening on port 55002 - this was automatically selected, and I’ve left well alone! No devices connect to the Three 5G Router directly, here, wired or otherwise, except for the Google Wifi Mesh itself. All devices then connect to that, wired, or wirelessly.
Kevin - I turn WIFI off completely on my iMac. It can ONLY access the network via Ethernet, connected directly to the MESH.
The Three 5G router itself is on the following firmware version (I think this has updated in the last couple of hours, actually, as the router appears to have restarted today): V1.00(ACBJ.0)b12. It’s the same model as Kevin’s.
On the topic of UPnP - the mesh network has this turned on, and I think it’s the MESH that handles this? I’m not using the Three Router to do this. The option is off, and I’ve done nothing manually to set up port forwarding in either the MESH network, or the Three 5G Router.
Poor connection issues
There was a point when I first posted in this thread where ARC seemed to be failing all the time in the way I described, and in the manner that Kevin seems to be experiencing at present, too. Now, it seems a rarer event that the Core locks up. Something changed, but it felt like it may have been outside of my control or not a direct result of anything I did, at least nothing obvious (i.e. I assumed it was either on the Three network or at Roon’s end).
I chimed in again, above, to highlight when a poor connection event resulted in the Core locking up, again. The subsequent event I referred to after that, which lead me reset ARC, didn’t require a restart of my Core.
ARC Settings and experience in the last couple of weeks
We’ve already touched on the fact that I’ve been leaving last.fm off.
Additionally, as a result of a recent mobile contract change to an unlimited plan at the end of October (to support use of ARC), I’m opting to use ‘Original Format’ whether I’m on Cellular or Wifi. I had wondered whether transcoding for ARC might add additional processing strain on the machine I run the core on, despite the robust iMac spec I happen to have. Thought I would mention that last point, in case it’s worth noting, too. I wanted clear the path from the Core to ARC as much as I could.
Finally, ARC continues to be working here. By that, I mean that while I encounter a poor connection message here and there, generally I am usually able to overcome these whilst on the move (ARC seems to behave at work, in the car and even on the train as a general rule), and I’m able to make a connection fairly readily when the issue does occur. I consider this progress from where I started! 
PS @benjamin - good tip on the playlist for downloads, I’ll consider that as an option. thanks.