I'm not sure what type of advice you're going to get here from seasoned players, but as someone who has returned to the game, my best advice is to seek out other forums or YouTube for build guides.

There's no single build which caters to every type of class. Instead, most of these guides offer recommendations to try and match the play style you want to stick with.

Some will recommend armor and focus on skills to level up quickly. Others will focus on how to balance for your play style. Some will even tell you the pros and cons between food/drink and potions. All of these are tips, not literal things to apply to your ability to play the character and style.

If you've been playing a while, then you're going to know practicing new skills is going to be paramount. By mixing them based on your play style will ultimately determine your role in PvP.

Your mention of OP is rather endemic of the game. You're OP against the AI, not against other players. Testing your abilities against AI is only going to get you so far. To learn more (and expect to die quite often), you need PvP practice, because no AI is going to match the abilities of a real person behind the foe you're attacking.

Hope this helps out a little, and I hope you understand why I can't really give you a direct answer. Perhaps this will help you answer some of your questions as you read/watch more.