As much horsepower as the 264 has, check out the Hornady 153gr A-Tip. It is nowhere near as popular as the ELD X or M, but it works well. If you’re a Berger fan, the 156 would be a mule deer/elk killer as well.
My uncle was born and raised in S ID during the heyday of S ID mule deer hunting and he shot a 264 for decades.
I could go on a rant about the 6.5 fad and the fact that the best 6.5 actually pre dates the 7mm Rem Mag and the 300 Win Mag… but I won’t. The 264 simply never took off because the barrels of the late-50s/early-60s sucked.
Good luck on load development guys. If you are hard up, I have an unopened pound of Retumbo in MT I’d off load for what I paid this spring. I grabbed some because it was available. Gave a couple pounds to my buddy, traded a couple for H1000.