How can I retrieve instance IDs for a certain mode, but not tied to a particular character?

Goal: I'd like to crawl through X number of instances of Y mode, to collect data and perform analysis on it. I am aware that I can pull a Carnage Report and just increment the instance ID, looking for a particular mode; I have found it to be very inefficient and brute force, and I'm hoping there's an easier way. Related to that, has anyone made use of the ActivityBlob or AggregateActivityStats endpoints yet? I've been unable to get them working. Cheers!