If deer was the largest intended game for the rifle, there's no way I'd be considering a 338. (plus it sounds like you have legit elk rifles if you ever want to hunt elk) It sounds like you have a donor picked out with a mag boltface. There's no real gaps in your caliber list. I'd pick something I already like or just want to try. I assume you reload. Maybe a 264/7STW/7RM? 26 Nosler? But you're right, there's nothing wrong with a 257Wby, just expensive brass.
