I know far too little about rifle cartridges to offer any useful advice as to which one is best for your needs. That said, there's an article in the latest American Rifleman about the Thompson Center Dimension. Apparently, there's one model that will allow both .270 and .30-06, and a different model that will allow .308, .243 and .22-250, of a total of 4 different models. (Don't ask me to explain the engineering. Swapping calibers within a model allegedly involves only a barrel swap, but beyond that, it's beyond the precipice of my knowledge.) As I recall from the article it will soon be available in left-handed versions, too.
A gunfight is not the time to learn new skills.
If you ever have a real need for more than a couple of magazines, your problem is not a shortage of magazines. It's a shortage of people on your side of the argument. -- Art Eatman