Your buddy gave you some good advice!
I may have misread your original post- I somehow took it as you were also wanting to get into loading your own AND beginning to shoot further out.
Your 300wsm is a great cartridge and has the ability to reach out there. An elk at 400 is no joke! If you want more, I certainly understand that. I would still consider learning all you can about hand loading though, and if not already doing so, fine tune a loads for that 300wsm you have. Maybe tweak the rifle, too, so that it is better suited for LR.
If you really just want something new, and don't want to load, any of the magnums already listed will do it. Even bigger, the 338 Lapua will pack a punch! Factory ammo is expensive, and currently it is quite scarce, so I still recommend diving into the deep end of the hand loading pool no matter what you end up with. Plus, it's fun!