I believe in buying the best optics you can afford and then sticking it on the best gun after that.
Your 300WM is plenty for 500yd elk. If it's that accurate, I'd spend my $$'s on some high end optics. My personal favorite is the Nightforce 3.5x15x50 with the R1 reticle, but for your rifle, I'd get the R2 reticle.
Then when you are ready to move up to 1000+yds, you already have the optics.
ps: For elk, get a .338.
If some is good and more is better, then too much is just right.
My mind is a raging torrent, flooded with rivulets of thought, cascading into a waterfall of creative alternatives