What is Sett's most reliable build when he mainly has to win long, central fights?

Bruiser sustain build to win extended fights.

When should you truly adapt Sett's build instead of simply pushing the standard bruiser route?

You should adapt as soon as the enemy draft stops you from playing the timing the build was meant to serve. If you need to survive longer, absorb more poke, or answer a specific engage pattern, the purchase order must move before you even think about the final six-slot build.

Which enchant should Sett choose when the real problem is mainly surviving after first impact?

gargoyle is often the most stable choice because it directly supports this game plan: absorb damage, engage and punish with delayed burst.

Sett Wild Rift Best Build Guide (Patch 7.2a)

Sett Wild Rift Best Build Guide

Bruiser build built around central punishment: Sett does not want to merely absorb damage, but to absorb enough for his W and control tools to become truly decisive. The premium build therefore looks for the exact balance between access to melee range, durability through the first burst wave, and the ability to turn a messy front-to-back into a fight won on one swing back.