The quest Ugly Baby starts Act 2.
Also may interest you point of no return to act 1: The Isle of Mists.

the last quest in Act 1 is called "The calm before the storm" completing this quest is the trigger that starts Act 2, as long as you don't start & complete this quest you will remain in Act 1

If I may ask, does the Act 2 ends the possibility to do anything else or can you come back later/post-endgame?

Yes.

So I need to stop before starting the quest. Thanks. I'm currently 26 and I won't like to end the game so fast.

No sense rushing thru as it takes a lot of enjoyment out of any game.

during Act 2 you still have the freedom to travel around & complete side quests/contracts.
After you complete & finish the main storyline the only quests that are not available are any quests that would have affected the story everything else you missed is still available in free play for you to complete when you choose to

In fact I'm not going to rush, but I was checking the level requirements to get the most XPs I could.
Now I just need some POIs on Skellige to spice the things up :D

Yes,but quests picked up in act 1 and carried over to act 2 are breaking the game for some.
They can't be finished properly,it's not like a lot of games where you can trigger quests and
do them later.That's when problems occur.

Are there dialogue options in the last few quests, such as Echoes of the past, Missing Persons, and Nameless that let you begin the next one later or are they immediately triggered and started one after the other? Because right now I am a little worried about beginning any oneof the quest mentioned above because i want to enjoy the game and finish as much as possible before it ends.

How many of the 4Million+ people playing the Witcher 3 is some? is it enough to assume that their problem is everybody's problem?

Yes the next quest is immediately triggered after completing but that doesn't mean you have to do the quest.
Main storyline quests are triggered deliberately by the game so you always have a clear path of progression but just because it's active doesn't make it a priority, you could have an active storyline quest but spend the next 10hrs completing side quests or contracts.

I left the "High Stakes" quest because I tried it once and got slaughtered, so I reloaded my save game to ensure I could try it again later. The quest remained in my quest log to do later for some time, but at some point it got moved to the failed-quests area even though I never went back to Passiflora to try it again because I wanted to build out my gwent decks more. So I put all of this time into mastering gwent and building up my decks and the game shitcans my chance to win the gwent tournament anyway for no apparent reason.

I did complete ugly man quest and all of Kaer Morhen. I've done practically every single white quest or area of interest on every map except for 2 "?" locations in the northern mountains of Ard Skellig which I can't figure out how to get to. I've climbed the mountains and stood on top of both of these ? locations indicating that they must be inside the mountain, but I've been unable to find a way to get to them. I found a cave not too far south east of them near a small town and found a secret area in the cave protected by several ghosts, of which I proceeded all the way through to find a doorway with a bricked in barrier I can't get past with Aard nor the illusion breaking device. I figured this passage might lead to the ? locations but no dice so far.

I've got one more witcher gear quest line to do that I am leveled up enough for, 2 that I'm not quite yet, and 2 or 3 other witcher contracts that are higher level than I am, and the only secondary quests are collect-all-gwent-cards, the statuette quest that I can't complete as I need to talk to Triss who vanished from the game, and the Isle of Mists quest which I'll be doing shortly as there's nothing else left to do. I wanted to do the High Stakes quest but the game robbed me of that and I'm a little disappointed.

That reminds me. Is there any way to complete the statue quest if Triss leaves at the end of Now or Never? I thought there was some mention of being able to ask Yen about it but I haven't gotten that dialogue choice and I've finished The Last Wish.

The quest remains active as it is possible to complete it later in the game, i don't like spoiling things so i'm not going to explain how, for now i would forget about the quest & keep playing as eventually you will be able to complete it.