You will not get a "mostly empty" Main Street, no matter which ADR you choose. Choose your meal based on preference for the meal if you are booking it for breakfast. If you are booking simply for photo opportunities, choose BOG. You are better able to dawdle while waiting for people who are all heading in the same direction clear a space for the pictures.
I also think that if park hours change, and they often do, you will feel easier letting the BOG ADR go than you will if you book CRT and start looking forward to that meal.
Be aware that no matter which ADR you choose, at 8 AM just about eveyone whose ADR is before 8:45 in all of those MK breakfast locations are allowed entry, so while you will not be walking with RD crowds, you will not be as isolated as when the only two restaurants open at that time were CRT and CP. BOG is a massive restaurant and is CS, so the number of people heading there is high, and I think that it at least doubles, if not triples the numbe of early entrance guests.