Each champion is a little different, but the core concept is trying to max out what your champions are meant to do, whether that be debuffing, doing DPS, needing to stay alive to protect the rest of the team with buffs and/or heals/revives, or a combination.
The issue with forcing sets is you often lose out on stats, which is 100% what you're doing.
As for keeping gear/what to look for, there's quite a bit of content out there.
https://youtu.be/lN2AxZGHMoU Nubkeks, who I'm a fan of.
HellHades also has some content on this subject, which I want to say was linked in earlier in this thread.