Honestly, I've redownloaded it on Xbox gamespass to get the achievements again after doing it all on PS4 and I can't get on with it. I've not got the camera settings how I had them on PS4 and it's irking me, they seem either too slow, or inverting in the wrong places, or something, but it's irking me more than it should. When I have it set up right, I love it, when I don't, it feels really janky.
As for whether it changes after 20 hours, not hugely. The more you explore the easier it all gets I guess, then you eventually unlock Skellige, the second region, but that's not drastically different beyond more annoying water and boats.
Like I said, it was only on Death March where I think the game really plays right. You can't survive a fight with a couple of wolves early on if you aren't careful, don't use potions, the right oil and the like. On normal, you can pretty much just Quin and dodge your way past everything and just hack everything apart when your shield is up. On hard you the combat is a lot more tacitcal (although still very dodge and quin based) It makes all the crafting (and therefore the picking up of endless stuff) necessary, so you really actually have to concentrate a bit on getting formula and blueprints and all that stuff. In terms of the story, it possibly is more immersive if you've at least played 2, but it's pretty solid, even if it's fairly linear. Geralt is Geralt, no matter what choices you make.
Most of the choices only affect the ending. My wife ended up nicely settled down with one of the ladies in her vineyard, everyone alive and the world at relative peace. I ended up all alone, with most of my friends dead and the Witcher equivalent of the Nazi's in power, so things can go a bit sideways!