As title asks, why the hell would Geralt go to Tousaint before finding Ciri. It's like Hild Hunt is after her, she's in extreme danger and while searching for her halfway your journey to find her you go on a trip far South to help like it's something more important... Clock is ticking and you go for a contract in the South. It shouldn't be possible chronically to go for that contract without making sure that Ciri is safe.

Nobody forces you to quit the main story line with Ciri.
In fact "they" might make you think twice of starting HoS and/or W&B simply by raising the entry level significantly.
Meaning: you will have a very hard time starting any of the expansions at a level that is sufficient for the main game.
So starting any expansion "early" is an option only - if you do not feel like it, don't do it. Simple as that.

we are talking plotwise here, not gameplay-wise

Why would he do Hearts of Stone as well..?
The answer is they are both meant for post game-credits play though they have a couple of small details / convos change based on what you've done or not in the main game. Finish the main game, it's been over a year since the release already....

I did finish it 4 times already. I do not like plot holes that's what I am saying. Or goofs

How are either of the expansions plot holes? They are entirely self-contained stories set at a player level far above that at which you would find Ciri. There is no plot hole. None.

Allowing you to go before finding Ciri to Tousaint is a plothole.

So anything besides the main story is a plothole. Don't make any secondary quests then...

A monster contract (side quests) doesn't involve Geralt traveling to another country and spend days and days to travel there. Also the other side quests of the game like Beat X, win gwent vs Y are things that could be solved very fast in terms of time. Also a monster contract is a must even while searching Ciri as Geralt should have some money in his pouch to live. So stop telling bullshit plz.

I think the main reason they allow that, is that many players still haven't finished W3 and CDPR does not want to lock out players who haven't. Simple as that.
However, I do think that B&W is intended to happen after W3's main quest.

How are you exactly locked when you just can start new game into B&W instantly?

Nonsense. By your logic, allowing the player to go to Visima or Skellige is a plot hole. If you press your mouse button on the Toussaint marker, that is YOUR decision, not a plot line decision. You seem to be utterly confused about what a plot hole actually is.

B&W is clearly set after the events of the main game, there are certain small things that happen to suggest it, even within the first hour or so. I've seen lots of reviews referring to it as Geralt's retirement, and it seems to fit that...Hearts of stone, despite being pretty awesome didn't seem to fit anywhere in the main story sadly, so I kind of took it as being set before the main story as it has literally no interaction with the main plot. It makes more sense in context considering some of the possible events of HoS.

So yeah, do HoS, the main game then b&w

Edit: obviously it's your choice, but I think CDPR are yknow, allowing you to enjoy the game as you wish

As many other posters have pointed out, it's entirely your call whether you choose to travel to Toussaint prior to finding Ciri or finishing the base game, and this is no more of a "plot hole" (to use your words) than Geralt deciding to become the boxing champion of Velen (and Novigrad and Skellige) or insisting on finding out who is compressed in that random jade figurine or what secrets lie in Aeramas the Tyromancer's abandoned manor prior to finding Ciri. Sure, you can rationalize almost any side quest or witcher contract by saying that Geralt needs coin, but that's pretty subjective.

Unfortunately, with open world or sandbox games it's extremely difficult to give players so much freedom to explore the world in a nonlinear manner while at the same time maintaining a sense of urgency when it comes to the narrative.