1. Why are the avionics on while starting?

Why not?
Because the electrical system in your Cessna 152 wasn't
designed to support that operation. But why should I sit there
burning 100LL at $4.09 per gallon so I can punch the
flight plan into the GPS while it acquires the satelites?
Fortunately I'm not flying a 152, so I can make it better.

2. ...Add an avionics switch.

Actually that would achieve the same functionality I have now:
The avionics are off during cranking.
It fails to keep the avionics up while cranking, and
also fails to use most of the battery's energy before
brownout. The latter has been offered as an excuse for
a 28V system, but adding a second battery costs much more
in complexity and weight.

I will get the dimensions, weight, efficiency vs load of the
12V supply and post. That may take a while as I'm putting the
engine back into our Maule this weekend (Yay!), and also have
some business travel coming up.