Use as fine of a crush as you can get, then adjust your efficiency to take that into account....on your next brew when you know the results. My first batch was based on a recipe that expected 70% efficiency but I got 80. After a few brew sessions with that high of efficiency, I started adjusting the recipe to take that into account. Since the crush is the main factor in efficiency, your results may vary from mine as I mill my grains very fine.
BIAB can be done exactly as you have done your batches in your mash tun. It can be done way differently if you want with full volume, no sparge, or a little less volume and a small sparge to gain efficiency and get to the preboil volume you want. It's still all grain but it just eliminates the mash tun cooler and false bottom/bazooka screen with a bag with such a large filter area that you can mill finer and not worry about getting a stuck mash or sparge.