I appreciate chatacter backstories because they give me hooks, and give me an idea of what kind of stories the players are hoping for. I don't insist on backstories. Totally up to the players. Sometimes players come to the table with zero backstory and have incredibly rich characters, while others have detailed backgrounds and very flat characters. I know I have had times where a character I've thrown together with little thought ends up being very memorable, whereas one with lots of background just fizzles. Just never know, which is part of the fun of the game.
As for metagaming? I think there is way too much hand wringing over it. I just assume my players know fire works on trolls, and if somebody remembers some more obscure weakness, no biggie. They have probably remembered wrong anyway.
And I change up monsters, so no guarantees.
Players get to know their GM, so metagaming is pretty unavoidable. If a particular GM expects players to talk first, they are going to learn not to attack everything in sight, whereas, attacking first, might be the best tactic under a different GM. Players are going to start unconsciously picking up on patterns, and not even be aware that their behaviour in Bob's game differs from Anne's.
I also find that certain "metagame" systems, such as Fate points, story cards, the escalation die, etc. actually enhance the engagement of the players at the table. To me, maximizing engagement as much as possible, is what is going to make the game world seem more real, and ensure players return to the table.
Really, the only metagaming sin I can think of, is a player reading a published adventure ahead of time, and acting on that knowledge. Or deciding to gather the ingredients for making gunpowder when it hasn't been invented yet, and said character is no alchemical genius.
We are sitting at a table, eating snacks, maybe drinking beer, dealing with phone calls, bathroom breaks, and getting distracted by such gravely important questions, like which is the best Marvel movie. Not going to worry too much over whether John's character insists that the hooded man in the corner of the tavern must be important somehow.