Could you drop the tenon thickness to 1" and house the brace as in square rule housing or haunch the shoulder line, the housing would give the bearing strength not the tenon.

Consider changing the brace length, dump the 45 degree angle and go with a 2:1 ratio, say..... drop two opposing braces down to 5 feet from the beam and run over on the beam 2.5 feet. Keep the other braces at 45s.

Size of the structure will hint at solutions. How many post do you need to configure this way? Replace just these post with 9x7s.

What species are you boring? Hardwood? Sounds like a feed screw issue. Did you try it in a few spots, you could have hit a void or pitch pocket which will stop the feed of the bit. Solid frozen wood could also be problematic, but so far this winter has been a dud, so I'm assuming frozen wood is not the issue. Sharpness may play a role, too. Does it grind through the wood or make a pleasant ssslliccing sound? Is it a coarse or fine threaded feed screw?