How do eggs sit with you? They have a lot of sulfur in the form of methionine, I think. I ask because this might be a helpful "poor man's" test to determine if the garlic/onion issue is sulfur or FODMAP-related...or both, I suppose, if you get some symptoms but not others.
Speaking for myself, I am a little leery of the idea of sulfur avoidance if the problem can be fixed by taking more molybdenum. I have a hard time seeing long-term avoidance of a nutrient as doing anything but causing problems.
I'm not saying that the body can't adapt to lower sulfur intake...the question (to which I do not know the answer) is how does it do this? Will you produce less taurine, glutathione, and metallothionein? Will phase II detoxification in the liver get backed up from lower-than-necessary sulfation? Or can we conserve sulfur in a more benign way?
Obviously I don't know a lot about this.
For what it's worth, I take 1500 mcg of molybdenum daily. I know some people consider that to be excessive, but it seems to work for me, and has for years. My last hair mineral analysis found no problem with molybdenum levels, and I haven't run into any "molybdenum toxicity" symptoms...in short, I think it's a good dose for me.
So there might be some other options.
I wish you the best with whatever experiment you end up trying.