First you need to consider buying from Masters Frame Shop Supply in New Albany. They are an A-1 bunch to work with. AFTER you buy from them, ask them any question you desire. If Lois doesn’t know the answer she will refer you to their framer. They are more than helpful and will even show you, their customer, how they do things and they've been around a long time!
Second, I think 80% of the miter is determined by the chop. Only 20% has to do with the joining method and moulding quality on most profiles. Nothing will make your joins more consistent than improving your chopping method. Dad bought a Morso when I was about 5 and I still use it today for most profiles. It doesn't chop everything but almost! Set it up right, keep the blades sharp, and I'll bet you will just about eliminate any joining problems you are having. It’s certainly much more accurate than your current miter saw.
Finally I want to commend you for wanting to master this aspect of the trade. In this day where every supplier is offering chops and joins, many have become completely dependent on their suppliers. Rarely a week goes by where there isn't at least one thread grumbling about the quality of chops, packing of chops, matching of chop rails, size of the chops and on and on and on. Much of the time it’s the truck driver or sales rep operating these saws. It's of no surprise to me that there is this much trouble with chops. Furthermore, I can't think of any possible way to order moulding less efficiently. Just to think, most of this could be avoided with what – an $800 machine? You’re on the right track!!!!
Carry on.