That's not really what sourcetracker is for, and it would not be clear if source features are actually specific to one group (e.g., they could be missing from some group members or just be more abundant in that group than others, I believe).
q2-sample-classifier could identify characteristic features but again it would not be clear if these are unique to your groups (the most important features probably are! But you would need to check to confirm). Still, it could be a fine place to start.
Honestly, I think that exporting and working in R or python is probably just the easiest thing to do since this really is what you might call a custom job. May be useful for others, though — feel free to share your solution or put together a method to contribute to QIIME 2 