Board Retaining Wall
2 x 12's across an 8' span will result in an unacceptable bow pretty quickly. Even the 4' span will bow, a couple of inches over time. I think in the long term, you would be much better off spending the $ and laying 4 x 6's for the wall, with 6' support spacing. I also would go no less than 3' on depth, to fend off the problem of long term leaning. Even the modest pressure from only the two feet of fill you are retaining, will have dramatic effects when applied 24 hours a day, every day, long term.