Tom, can you post a picture of the anode to show just how much it has
deteriorated since March?

Your definition of "deteriorating rapidly" may be what could be quite
normal, as Chuck pointed out.

The boat is in storage down at the bay but I'll try to remember to
take the camera next weekend. I was concerned and the dealer was
amazed at how fast it had eroded so I really think it's excessive
(although not excessive when you consider what it's protecting).

I would definitely trim the unit out of the water when the boat is docked as
that would be the best solution. Raising it is a one minute project (not
labor intensive other than to your thumb) and it will keep you lower unit
from looking newer.
Not a bad idea. Between the jack plate and tilt I can completely clear
the water.

As others have said it is also a good practice to turn the battery switch to
"off".
True but the dealer mounted the switch against the transom under a
hatch in the rear. When the bimini is down (which is most of the time)
I have to loosen the strap, move it forward, open the hatch and reach
down from a kneeling position to change the switch. A real pain but
worth it to save the boat I suppose.