Figured I would update with a couple other options I found. First is the Specialized Sequoia, which I must have just missed because I don't really pay attention to big S. Cool bike, seems pretty capable on/off road. Then there is the Wilier Jaroon, which I admittedly saw the name a few times but didn't think Wilier would make a steel adventure bike... guess I was wrong.

If anyone else knows of any flat mount frames, road or gravel/adventure/cross, lemme know!