Loading for the 338ARC and 9” barrel

I’m having a little trouble with my rig. This is my first subsonic-only gas gun. AR15, 9” Faxon barrel, carbine buffer tube and stock. I’m running a SiCo Hybrid 46M but could run my Omega 9k but haven’t tried yet. I’ve tried a 2.95oz, 3.75oz, 4.25oz and 5.4oz buffer. The 4.25oz seems to perform the best but the brass is ejecting well but at 5 o’clock.

I’m getting 980fps and sporadic hold-open with 14.5gr of CFE BLK and a magnum primer. 300gr SMK. I read around and it seems like I’m nearing the max charge weight for CFE BLK.

Would A1680 be a better choice? I heard it’s dirty and would hate to fill my can with carbon.

Any other suggestions?