I want a barrel to last me 8k rounds or so and I'm invested in 308 already.
If you just like .308 for whatever reason, that's cool. But do the math. Barrels are cheap, ammo is expensive. Even if your barrel lasts forever, missing will cost you.
There is no more cheap surplus .308 hiding anywhere. Factory rifle rounds in almost any caliber these days cost ~$2 a round if you're lucky (even the crap). If you reload and buy in bulk you can cut that in half (so ~$1 per round), but that's about it.
Using 6.5CM as an example (since it's already been mentioned):
6.5CM is 30% or more flatter, and 40% or better at beating wind than .308. So at only 300 yards, with 10mph of wind, if you blow your wind hold with the .308 you'll miss a 3moa/10" plate by a few inches, while you could forget to hold for wind all together and still get an impact with a 6.5CM. The performance gap gets wider the further you go out.
So after shooting 8k rounds with 6.5CM you'll be on your 3rd barrel, and if you went with decent X-caliber prefits for ~$400 a pop you'd still be under $1000 in added cost (that's including buying a barrel-vise and action wrench). You could even spin on premium barrels for maybe another $500.
Or you can shoot .308 for 8k rounds and not have to buy any barrels... Just don't miss, because if you miss 30-40% of you shots, over 8k rounds that'll have cost you ~$2400-3200 (assuming you can reload for $1 per round).
Life is short and rounds ain't cheap. Might as well load and shoot a cartridge that you're ~30-40% more likely to hit with. Pay a gunsmith a few bucks to take your first barrel off and after that you'll never have to see him again if you don't want to.