OK, per the thread on "Question on grounding rods", I'm supposed to use
8-foot grounding rods rather than twice as many 4-foot rods. I can see
that.

But, we're built on a ledge, one big wide rock that stretches from the
seacoast to the Vermont state line (that's why they call New Hampshire the
"Granite State"). I can get three feet depth at best before running into
solid rock. An electrician friend of mine says I can drive the ground rod
in at a 45-degree angle, but that would still run an 8-foot ground rod to
a depth of more than 5.5 feet below the surface.

Can't be done.

So, what do I do next?