Comparing 20" velocities in both 6.5G and .223 (with a sufficiently heavy 69-77gr) I found no point to the 6.5G UNLESS you are looking for energy. My 20" with an adjustable gas block and suppressor launches bullets in that weight from 2850-2960fps, accounting for less drop than the heavy 6.5G loads I looked at. Wind difference at 10MPH full value was minimal as well, maybe .2 MIL. As you're not looking for energy, there is no benefit to 6.5G IMO.
.223 will teach you about wind for sure, especially out at 600yd or further. I've found having enough magnification and spotting misses to be the problem, especially around 800yd when using the 'typical' tactical style scope with ~10-12x magnification.