The vital skills require you to have to rank up the class 3 time to reach the full 15 level potentials else wise the skill will become weak. The class that you only rank up twice or even once are consider filler classes, excluding the highest rank ones. The filler classes are mainly used to get access to filler active skills during cd, for buff/debuff, or mainly for utility skills.
Also, you need to have a clear idea on how you want to fight with your character, filtering out unnecessary skills that does not fit your style.
With these in mind, the amount of combination to pick is easily reduce to a manageable amount.
Min/Max rotations is a completely different story, but from what I know, most true Min/Max player enjoy creating their build to find the optimal specs and should not really have a problem complex skill system. In fact, they love it, because they like to build brokenly OP characters.
Also forgot to mention, depending on the type of gameplay and mechanics the game uses, maxing out skill level with limited skill points is not always the best.
