Per Don Y:
Ethernet is also a potential problem due to exposure to lightning strikes.
Consider running optical fiber to bridge buildings. Or, protect the
ethernet drop accordingly.

OK, now MakOlber's comment is starting to soak in.

Since I know nothing about fiber - but have experience with those $150
radio links described previously - it sounds like I am going to retire
the Ethernet and dedicate that conduit to coax.

The remaining question seems to be grounding.
--
Pete Cresswell