D5 is the highest Difficulty. And it is all you ever need.
If there are several reasons to raise the Difficulty, then D5 is the limit, no matter if there are more conditions to raise it.
If you have a character with 2 Mental Trauma (+2D), not treated, trying to spot something at Long Distance (+2D) during an overcast night (+2D), that does not make a D1 Observation test a D7, but still “only” a D5.
You CANNOT succeed at a D5 test without any additional means by only rolling 2d20.
Either you roll additional dice (for Momentum or Doom), or spend a Fortune point (which counts as an additional d20 set to a 1), or have some Talents that allow you to lower the Difficulty for a certain test under certain circumstances.
That is, why D5 is still the maximum Difficulty.
That said, if you do have circumstances that raise the Difficulty, like in total +6D for example, and you have a Talent that reduces the Difficulty by -1D, you calculate those before applying them to the roll. So in this example +6D -1D = +5D, still makes a D1 test to a D5 test, not a D6 test.
There are some Talents, that allow reducing the Difficulty by more than -1D, for example the Tracker (Survival) Talent that you can take up to 3 times, reducing the Difficulty by -1 per rank, to a maximum of -3D. Those make the difference for tasks that normal, untrained or lesser trained people will most certainly fail at, but a very competent PC might succeed - often with ease.