50mm objective is awefully big optics. Nice to have, but 40mm with a 30mm tube and good glass would probably be more than sufficient for what you are looking to do.
7mm RM, 30-06, 300 WM would be great options. Even .270 could work fairly well – depending on the range you plan on shooting a moose at – although it wouldn’t be my first choice.
Are you recoil sensitive? If so, maybe go with a 7mm RM or 30-06. Is the gun going to be heavy or light? Factor this in with recoil considerations.
My 1st pref would be .300 Win mag. Would make a nice all around gun for those animals, plenty of power. Plenty of factory load options if you don’t handload. Less expensive than UM’s for example. Consider .325 WSM or .338 WM if you might use it for dangerous game (i.e. brown bear) and don’t mind recoil. (Be honest when considering recoil – if you lie to yourself and say you can handle the mag recoils when you really can’t, you’ll flinch every time and accuracy in the rifle/cartridge won’t matter much.)
Ultra mags are great on paper, and I wouldn’t mind owning one. But the ammo is quite a bit more expensive, and they really only get you a few hundred FPS muzzle velocity vs “standard” mags. It’s nice, but it’s a minimal end result difference, with bigger recoil and higher priced ammo….
I would heavily consider recoil and rifle weight (are you going to be carrying it all day long up/down mountains?) before deciding on a caliber…
Good luck!