One technique when using malt extracts is to add most of them about 5 minutes before the end of your boil. This helps to keep the color of beer lighter as well as keeping the sugars more fermentable (i.e. keeps the sugar from carmelizing too much).
Proper yeast amounts is a good thing, never a bad thing
. Although, one sachet (11g) of dry yeast per 5 gallons of 1.055 wort should be enough/plenty, especially if you rehydrate it. I understand the reasoning for rehydrating and believe that it makes a significant difference, which ISN'T to say that you can't get good beer from direct sprinkling on the surface of the wort, just that you are giving the optimal chance for the individual yeast cells to survive in the wort.
Prior to oxygen, I used to shake the sh*t out of my carboy and had good fermentations. I would not say that oxygen has made my beers better - just less effort. But I would really get a good shake, as best I could, for a few minutes. At the end of shaking I would have inches (that's lots of centimeters for you metric folk
) of foam prior to pitching yeast.
There are a lot of posts on here of folks to get stuck at about the 1.020 mark (if you search for 1.020). I bet reading through those threads would give you plenty of ideas to try on your next brew.