Is it cowardly to not tell Rita 'hey, by the way, I'm planning on ditching this place the first chance I get?' Absolutely. Does Strange really not want to fight this fight at the moment? Also absolutely. So, he pins his hopes on Rita accepting what he does when he does it and then goes back to talking shop.

"It would be ideal if we don't have to rely on too much of anything aside from pure magic. Unsurprisingly, most of my ideas revolve around the Silver Mirror, but I'm loathe to put too much stress on the thing."

And not just because the Silver Mirror is literally Strange's bones these days.