Re: who makes 9/16 24thread muzzle brakes
Brownells has a bunch, but they are all pepperpot style just like AA's. With one exception, the R3e2c, but that is for a .30 cal. I'm pretty sure it won't be real effective for the Grendel. It's also for higher volume than what the Grendel case holds. Yankee hill makes a linear slotted one that would work well if you just want to bore it out for 6.5 and ream and re-thread it 9/16x24 from 1/2x28.
FWIW, I couldn't find any lateral slotted types either. Not without having one made.