It can get bewildering, but you are on the right track. That’s why about 7-8 years ago I quit paying attention to energy measures. I focused on selecting a bullet of a size capable of cleanly killing my quarry and delivering it to a lethal spot with enough velocity to work as designed. I generally pick my max range for a given hunts environmentals using the bullets minimum velocity to perform, and then add 100-150 fps just cause.
So for me the same cartridge can have very different ”lethal“ ranges based on the bullet selection. A Barnes TSX vs Hornady ELD-X being an example. More specifically, we’ve had very good results from new hunters for well over 15 years with a 260 Rem and 140 gr Core-Lokt to 300 yards or so. The recoil is more of a push and they shoot it well. So your 6.5CM will take elk, just know your bullet and plan the max range accordingly. Good luck!