Doc here,
Check your oil, Is it Milky or Green?
Pull the plugs...Crank it over, does it "Spit" water out the plug holes?
Does the Starter spin fast when trying to start, a few revolutions, and then kinda lock?
All signs of a blown gasket or damaged casting material. Do a compression test..are a few low next to each other? Bad gasket..
ODD, though, That is is , as you say, not heating up and still using that much coolant.
Doc