You will not outgrow a good 223 bolt gun. The best field shooters I know shoot 20-1, 223 to anything else. I disagree heavily about wind being a detriment to the 223 past 400 yards- yes, if your sole concern is hitting small targets in wind more than learning- but then why not just shoot a 35lb 6 dasher PRS rifle and get even more hits?
Most things that need to be practiced with, can be practiced with bulk ammo. 55gr FMJ will stay on 12” plates to past 400 yards, and most people lack gun handling the most, which is where volume comes in. For anything precision related (sub 2moa) Hornady frontier 68gr and 75gr HPBT works just fine as is $12-14 per 20 rounds- no 6.5cm is available for that.