I'm going to take a stab at answering your question, and also give your post a little bump so that others have a chance to see your post and respond to it.
BTW, noticed you were new. Welcome to SMF!
So, most sources recommend using pork fat rather than beef fat in sausage for a couple of reasons. One is that the pork fat has a lower melting point that beef fat, so it has a nicer mouth feel and does a better job of helping the sausage stay moist. Second is that the pork fat has a milder taste than beef fat....supposedly beef fat is strong enough in taste that it might overwhelm the venison taste. I haven't tried it myself to be able to comment to that. However, when I make venison breakfast sausage I use 80% venison and 20% pure pork back fat and this works great for us if the venison is mild in flavor. So it seems like 80% venison and 20% straight fat (be it beef fat or pork fat) would be fine for bulk sausage, barring any flavor change from the beef fat.
In regards to the rendered fats vs. the unrendered fat....I have never seen a sausage recipe that uses rendered fat (tallow, lard). Always the fresh unrendered fat. The rendering process separates the pure fat from the cell walls, connective tissue, etc. So it would definitely change the mouth feel, and I'm guessing that the rendered fat would melt and leak out of the sausage when you cook it leaving nothing but a crumbly texture.
Any, FWIW, hope this was helpful.