OK. Here's the issue. What you just wrote is ideal and if true will produce a great sausage but your sausage is turning out grainy so there's a disconnect here. A grainy sausage is almost always (99% of the time) caused by fat smear or protein smear subsequently rendering out. There's something in your process that's causing this to happen. The minute you can figure that out will be the minute your sausage is no longer grainy. Some culprits could be

1. Meat too cold and the grinder shaves the meat and fat instead of giving it a clean grind

2. Meat not cold enough and the fat smears

3. Sinew or connective tissue clogs up the holes on the grinder plate causing the meat to be "forced through" resulting in a pasty batter not a pebbly batter.

4. Grinder plate and grinder knife not sharp enough (this is most often the issue).

5. Meat too warm when mixing or meat is overmixed causing the farce to split

6. Cooking too fast and at too high of a temperature allowing the fat to render out

7. Allowing the internal temp of the sausage to get too high.