I lean 8”, but depending on the elevation and temps you’re hunting at, you might give up a little BC with a 108 elite hunter, or 115VLD.
Any chance you’ll shoot varmint bullets? I’ve blown up Sierra Varmint bullets, and Berger hunting bullets in an 8” twist 6mm Remington. When going with a tight twist you have to decide if you’re willing to give up on slightly lighter bullets and high velocity. Because I’d rather either go with a 6mm RemAI and 115’s or give up the 108’s and 115’s and gain the velocity, I’d would go with an 8.5” if I had it to again. If I really wanted a 6mm mule deer gun, I might go 7.5” and shoot 115’s.