I think one reason why BMW pressurizes the gas tank is to help prevent the more volatile components of the fuel from evaporating. Yes, there's a vapor recovery system, but with long-term EV use, and no fuel use, that would get saturated. Anyway, if you're worried about it, you might try some fuel stabilizer. If you're really worried about it, you might find a place where you can buy MOGAS (unleaded, aviation fuel), but it will cost you. That has a bunch of stabilizers in it already and is alcohol free. Where I live, I can't find any car gas that doesn't have ethanol in it.
2011 535i x-drive GT, 2014 i3 BEV
Soon (hopefully!) A 2021 X5 45e will replace the above