I've played this game until I've entered the first city (after the stuff of the beast) and did a couple of sidequests, but the melee combat Is either incredible overpowered, or alchemy (with the exception of 1 or 2 potions) and the other stuff, like oils and bombs are totally useless (I've played on the normal difficulty).

I kinda wanted to try the game where the alchemy, oils, bombs and etc is really a necessity, where you need to use a lot, and the hard difficulty sounds like that when I'm going to select the difficulty.

My question is: it's really difficult through the whole game, or it is possible to breeze in the end game on the hard difficulty? thanks.

If you find normal difficulty too easy, then it's probably best to increase it to hard. Keep in mind, though, that at hard difficulty you don't get the flaming sword help for knowing when to attack again for getting your sequences right.

I've not played through the whole game on hard difficulty so can't say anything about the difficulty curve throughout the game. But if you found normal too easy or boring, it's probably worth a try on hard.

I have played the whole game on hard. The earliest sections are toughest, with the boss battle at the end of Chapter 1 being one of the hardest fights. Later, when Geralt is more powerful, things are a bit easier. But I definitely found I was using alchemy a lot, throughout the game. I would have to prepare appropriate potions and oils before most fights.

So restarting on hard may be more enjoyable. There are also some user mods that increase difficulty, but I haven't tried them myself and I think they make some pretty major changes to the game, so they may not be appropriate for a first playthrough.