I have 3 AC HDs and one AC lite.
I also created a specific WLAN that only broadcasts on the AP closest to the hub and only uses the 2.4GHz radio. I took these precautions because my Tempest is on top of a tower well out of reach and to bring it down is a major PITA. It was running reliably a few days on a chimney mast beforehand.
I also connected my hub to an Anker 15Ah power bank and use the original hub power brick to keep it topped off. This way I can move the hub around if needed without interrupting its power supply. My Tempest unit has almost 15 days of uptime (counting since it was switched on from taking out of the box on December 5th) and the hub just over 8 days now (counting from when I connected it to the power bank). I guess I’m lucky.
Your RSSI values look excellent. Not sure why your hub is rebooting, that would be annoying AF! You said it was your second one that’s doing the same thing?
Maybe time to move its physical location. Your signals have lots of headroom so moving it further from your AP as well as Tempest should not be an issue.
And yes a POE powered HUB would be outstanding! Especially if the POE ethernet connection was an active, wired interface eliminating the need for wifi.