Feeding- most alliums have shallow root systems and won't 'mine' nutrients from sub-soil. I till in composted manure at planting time and then side dress the overwintering alliums again in the spring. Spring planted onions don't need more that the single application.
I think a well balanced fertilizer with ample N for good top growth early on, then decent Phosphorus for root growth later. If you need more detailed N-P-K info, I can't help much.

Watering- alliums like regular watering and don't like to dry out at all while growing. Only when they are finishing up is it safe to withhold water.

Planting depth- onion seedlings, not below the point where the leaves are emerging, sets- 1 inch deep.

Soil- well drained, and fertile. Emphasis on the well drained part, onions and garlic will rot if left in standing water for long.

Ph in the safe zone you mention.

Hope this helps to get you started.

-Mark

Thanks Mark.
The information you gave is very fundamental
You pointed out to a VERY important issue and that is the SHALLOW root system. For this reason, alliums require different watering and feeding scheme than tomatoee, for example.. That is, at the most, moisture and nutrients are needed in the top 10" of the soil, max. So I think that top dressing with granular slow release fertilizers is best. And also because of shallow root system, more frequent watering should be needed.