As far as odds of failure vs. slight success vs. complete success, the trouble with RTD is always that the odds of one failing are constant. In this case, there is a 1/3 chance every turn you perform a "significant action" that you fail miserably, a 1/3 chance you'll accomplish what you set out to do, and a 1/3 chance you will succeed, but maybe not as much/in the way you expected to.
I'd say maybe you need some sort of "action modifier" (e.g. taking aim with a single shot improves your chances of succeeding whereas firing a flurry of bullets has a slight penalty to chances of success) but of course that could get REAL complicated.
Alternatively, you could increase the sides on the die, as more potential outcomes allows you to refine your odds. A 20-sided die allows you to have down to 20ths of a chance, like a natural 20 would be an "extra special success" that may seem supernatural, but is really just a stroke of extremely good luck. Here are some ideas I came up with for outcome lists:
20-sided die (Very small chance of "supernatural" rolls):
20 - "Extra special" success
17-19 - Complete success
14-16 - Good success
11-13 - Slight success
08-10 - Penalty-less failure (neutral)
05-07 - Minor setback
02-04 - Significant failure
01 - Unbelievable failure
10-sided die (No "supernatural" rolls):
10 - Complete success
8-9 - Success
5-7 - Slight success
4 - Penalty-less failure (neutral)
2-3 - Failure
1 - Complete failure
Feel free to use my ideas and change them up however you want. It's your Game after all.