Smoke 'n Sanity makes pretty good onion and garlic salt subs. I don't find them to be particularly salty, for what it's worth. Their ranch is pretty good. I've tried some of their other seasonings and they range from good to not great. I really liked the teriyaki the first time I had it but it's very strong smelling and I can't stand it now.
Scallion greens are a good onion option. I use dried chives in place of onion flakes. Ginger isn't an onion sub but does compliment onion and garlic in a lot of dishes.
Celery is another moderately high FODMAP food that is a main component in lots of dishes (mirepoix). Fody makes a really great vegetable stock powder that contains celery and carrot notes.