Instant Enemy does not come online until 10th level. A lot of games don’t get too much higher of a level, so focusing on a single enemy is rarely going to be that good of an idea.
Have a lot of medium bonuses instead of a two maxed out will usually give better results.
10th level is coincidentally also exactly when you make the choice of +2/+2/+6 or +2/+4/+4. So before level 10 you don't actually make any decision about spreading out your bonuses or not. And the issue about spreading out the bonuses is that other classes' accuracy steroids will catch up if you do so, making your FE just a more limited version of those.
A +4/+4/+2 spread at level 10 means that the Slayer (or Inquisitor) is just 1 point behind your maximum but much more flexible.
A +4/+4/+4/+2 spread at level 15 as a Ranger just means being on par with their accuracy boost, but way behind if you're not fighting your FE.
This is why Rangers trend towards stacking bonuses.
I don't doubt that your NPC sheriff was capable. But he was put by you, the GM, in a location where his prioritized Favored Terrain and Favored Enemy will always be highly relevant. PCs don't have that advantage.
Re: Combat Styles
I'm aware that there have been additions, and some styles are better than others (as mentioned), but generally they don't allow you to meaningfully cheat prerequisites. Let's take a closer look at the Cayden style:
2nd: Catch Off-Guard, Combat Expertise, Combat Reflexes, and Weapon Finesse.
6th: Improved Disarm and Lunge.
10th: Disarming Strike and Improved Critical (rapier).
The bolded feats are those you already fulfilled the prerequisites for, making them equivalent to bonus combat feats if you actually wanted those feats.
Improved Disarm at lv 6 is actually pretty late, a Fighter can at this level pick up Greater Disarm. If you wanted to focus on disarm, chances are you've already taken Imp Disarm before lv 6 (qualifying through Dirty Fighting nowadays), making it worse than a bonus combat feat as you must retrain to make use of it. Furthermore, if you wanted to keep branching out in the disarm feats (like taking Wrist Grab) then cheating the Int 13 prerequisite or Combat Expertise doesn't help you at all.
And the value of cheating prerequisites of Disarming Strike at level 10 is inversely proportional to how much you've already invested in disarm. Which you've likely done if you choose a combat style based on disarm.