Bench shooting to 500 Yards can be done with almost anything

but i get what you're saying.
I have a T3X CTR in 6.5 PRC It was my all around hunting right for a while. I however like to dabble and play with anything I can get my hands on.
I do agree with RJ and think a .243 win would be a fantastic choice in the 6mm caliber. Heck I know of a marksman that used a 22 ARC on a moose this year without issue. (Not sure the distance.)
With my 6.5 PRC I have 3 loadings. 127gr LRX, 150gr ABLR and I am working on a 120gr CX. The LRX and CX are limited on range based on muzzle velocity and minimum expansion velocity on target. The ABLR is a more versatile choice and I may switch to the 142gr option due to availability. It can typically go a little further before it reaches an expansion problems.
I also shoot a 6.5x55 Swede which I load with 147gr ELD-M and 120gr E-Tips.
I recommend it as a cartridge in BC that will do almost everything. I don't really like taking it into Grizzly or Brown Bear country but would if I was goat or sheep hunting.
Based on the info given and what game you're looking at I would says a 6.5 CM, 6 CM and .243win are great choices. The 6.5 PRC is great as well but being a magnum it adds powder and recoil. I owned a Tikka T3 varmint in .243win and it shot lights out with everything typically .6" 5 shot groups.
You are also correct that you're going to get allot of answers on this type of thing. Everyone has a different opinion or how they choose cartridges. Velocity, Caliber, Magnum vs Non, Recoil etc.
RJ always has good input as I find there really aren't many cartridges he hasn't shot or owned. Me I choose to use math. Pick a game, pick a max distance, pick a projectile then see if the cartridge can be loaded to that combo. If not I move onto another cartridge.
Quick EDIT: this is all factory rifle conversation with factory ammo availability. If you go custom it opens up more doors but also means hand loading yourself if you're into that.
B