Formula (as well as solid food) stains will sun out but not as well as breastmilk poop. Breastmilk poop stains are really bright and will literally disappear in a matter of a few hours (if not less) in the sun. Once you're past that stage it works but just takes longer and may not be quite as complete.
You probably will need to start putting the poop in the toilet (using whichever method works best for you, lots of options between spaying, dunking, using disposable liners, fleece liners etc...). It's hard to say when exactly that will happen as I don't have personal experience with half/half but you'll probably know. The poop will smell more and change consistancy (probably going towards the peanut butter type texture kind of thing).
If stains really bother you, maybe consider fleece liners. They'll keep most of the poop off the diapers to at least minimize stains plus they make poop 'removal' easier since poop tends to stick to fleece less than to the terry of the diaper. The stay-dry benefits are also nice and homemade fleece liners are inexpensive and very easy to make (literally just cut to the size you want and you're done, no sewing needed as fleece doesn't unravel).