Wow this is a big question. I'll answer part of it to get you on your way. Sorghum won't usually be too tart. It'll have some off flavors and that's why you might want to substitute brown rice syrup. If you were to substitute BRS for sorghum, you'd end up with a drier beer, i.e. a lower FG. The flavor would be more neutral and cleaner. So you could go 50/50 if you wanted to keep some of sorghum flavor but reduce the twang. Personally, I'll never use sorghum just because it tastes so strange to me. But that is aside the point. Another option is tapioca syrup. Briess makes one suitable for brewing. It makes for an extremely mild base to raise your OG. From there, it's easy to add toasted grains to achieve a certain flavor. Any malted GF grain is going to be fairly neutral without much malt characteristics, which is what most are going for in trying to replicate regular beer. Malted quinoa is pleasantly sweet and when roasted can produce most flavors you're looking for. I've found ~30 lovibond produces a nutty flavor. Roasted beyond that you'll get anything from chocolate to acrid, burnt. With rice, you'll get a variety of flavors. All rice can be roasted to various levels as desired. Red rice will produce a nutiness. As for the other rices, I have not experimented too much. Buckwheat can also be used as a base malt, though I'd warn against using 100% of it. Again, you can roast it and produce a grain that tastes akin to Grape Nuts, a cereal. If you've ever had buckwheat, it has a very distinct flavor, that of... well buckwheat. So if that's what you're looking for, then go for it. I can't comment on other grains as I don't have enough excperience. Another very useful tool in the arsenal of a homebrewer is buckwheat honey. If used in the right amount, it can bring a malt flavor that is hard to achieve otherwise. As for candi sugars and other sugars, it'd be the same as normal brewing so I won't go into that. Hopefully someone else can fill in the gaps.