>>>>A few things you can do is to search by segment (DTW-NRT-BKK), instead of just the departure and arrival cities.

The trouble with searching by segment (i.e., DTW-NRT and then NRT-BKK) is that fares, and class availability, are constructed by origin and destination (i.e., DTW-BKK). If you look at individual segments you can see wildly different availability in different classes than if you pull up your full origin and destination, because the airline will be willing to sell different capacities depending on markets. The airline booking systems USED to let you pull up segments, snag cheap fare classes, and then price the entire journey at the cheap price (if going DTW-STL, you used to be able to pull up NRT-DTW-STL, and a cheap letter would suddenly be available for snagging, and you could then just cancel the NRT-DTW portion), but journeys and class availability have been "married" now for about 15 years.