So I think I may have found the culprit! It wasn't a hub issue or a device problem. After observing all three of my hubs, including my "control" C-5 with just one device, rebooting every few hours for days, I started looking at my Unifi network. I seemed to remember an old post by a Developer that said the hubs would crash if their network stack was flooded. There was also some reference across other posts about mDNS being an issue. One of the Unifi network updates added an mDNS Proxy Gateway setting. All my IoT devices are on a dedicated VLAN with firewall rules to prevent inter VLAN traffic. I noticed the my Default, Cameras and IoT networks were enabled for the proxy Gateway. In desperation I removed the Cameras and IoT network from the mDNS proxy settings. For good measure I turned off mDNS in the IoT Network. Immediately Java memory across all three hubs stabilised then climbed. So far I've had 26 hours without a single reboot.
(You can see on the graphs the exact time I turned off the mDNS settings.)
In addition my LIFX bulbs which I thought were malfunctioning (I even replaced one) stopped continuously disconnecting and reconnecting to the network. All my hub mesh enable/disable messages went away AND my Synology Surveillance Station setup stopped having random camera drop outs! Phew.
I don't know what the mDNS proxy Gateway is meant to do but all it did for me was screw up my whole household.
I've since setup my "control" hub on a dedicated test network. I'll reenable mDNS on that network and see what happens.