My 2 cents, the 6.8 is dying I think. I'd go with 300 Blackout/ 300 Whisper.
You might think that, but it's actually more popular with the hunting crowd than defense and "tactical". It's a great hog round, actually Bill Wilson's favorite. The 6.5 Grendel would be king, but it's designer killed it's potential for success by refusing to share proprietary information and trademarks. The .300 BLK round is lacking in ballistics at range, so hunters aren't embracing it the way they do the 6.8.
I don't think the perfect mid-bore AR round has been developed yet, but until it is, the 6.8 will do good enough.