Yes do grade 2 until 18, once you hit 15 do the next level of leves. This is the best source of sp, as you can mine more at each spot and have an ability to find the next spot. If you move to grade 3, you have less mines per point, make the nodes degrade faster and have no idea where the points are to go to.

As mentioned wait till 43+ then you shall feel the pain. 48+ 100k to level and still no higher nodes XD