Yeah changa in a bong is a great way to go
I'd use something else besides marijuana...at least initially. It'd be nice to have as an option later- depending on how weed affects you. But then again you can always just throw some weed in with the changa/dmt if thats what you want. In any case, i would get a feel for how dmt/changa is on its own first.
Maybe use something like caapi leaf, mullien, etc, and make changa. For some of us, using only weed with dmt so much at the beginning of our work with it led to a bit of a crossover affect; where the anxiety that can come from the weed high sort of imprinted onto the dmt experience. Not fun, and it can be sort of difficult and take time to shake off completely.
Everyones different though...and others dont prefer weed + dmt for other reasons; for example it clouding the experience/recall
I dont usually mix them, but when i do its very very carefully and usually in low doses. For some reason if i smoke weed after dmt, there is way less anxiety than if i'd just smoked weed by itself. It makes weed much more enjoyable, and from that point it usually mixes great with dmt for me if done right