"pre assembled" being the operative word!
So if we are arguing semantics, the best "pre-assembled firearm ammunition" (definition of "round" in firearm parlance, per above) for a .300 win mag is the kind that is designed to be fired in the .300 win mag.
The best bullet could be from Hornady, Nosler, Barnes, Berger or bla bla bla. The best powder could be any number of them too. So what is being asked here? I'm still confused.
I think what you are asking is "What is the best load recipe for a 300 Win Mag cartridge you have found that works with a stock Tikka T3x 300 Win Mag, given it's magazine limitations?"
And there is no answer to that question, other than the "one that is safe and shoots most accurately in your own unique firearm". The answer to that question from someone else with a different rifle, even if same make and model, is largely irrelevant. I don't really even understand why people ask. It's kind of like asking about the best boots. It just depends. And you'll never know until you experiment and find out yourself.