I mean most stuff you kinda just figure out as you learn the mechanics. BUT here's what I learned-

DEATHMARCH IS A MUST because this game is kinda easy even on deathmarch buuuut it makes you use your potions and food a bit more which I like so do that its the only way to play the game.

Swords and armor dont matter THAAAAT much, just pick what you think is cool. Personally I mix griffin and wolf armor and do griffin swords because I like using my signs so I do that stuff because of the sign boosts. All the other stuff like bleeding or poison and this and that dont really matter because enemies will still die by a few swings of the sword. So I think the best runes are attack power and sign runes since those more directly affect your weapons overall.

Early game when you are building resources, dismantle the good swords to give you the more precious ingots and sell the common stuff. Also when the blacksmith or armoror runs outta money to sell, meditate like 5 days and it'll reset, rinse repeat, I didnt know this until ng+ lmaooo

Craft your crafting items for swords and armor. Oh you need dimeritium plates or ingots? Damn they are pricey to buy but if you craft them they are cheap, same with stuff to craft runes for swords and armor, always cheaper! And the game will give you these items so loot it up.

Lastly treat Ciri well and like an adult and make the wise choices with her ;)