Doesn't Afghanistan want a part of Pakistan.
No, I don't think so.
Isn't the main reason they fund the Taliban was because they don't want them to attempt to reclaim the land so to keep Afghanistan fucked up and unstabilized. The same reason that a sub group of Taliban has premise in Pakistan is being the group that wants to be part of Afghanistan. If by some miracle that country becomes connected and stabilized that would be a huge problem for Pakistan.
I think it's more straightforward than that - Pakistan wants Afghanistan under a friendly regime, they don't want Afghanistan under a regime that might favour the USA, China, Iran, Russia etc. against Pakistani interests. The Taliban are their allies - part of the link here is that there's a ethnic dimension, because the Taliban are primarily Pashtuns, and Pashtuns are also a significant proportion (10-20%) of the Pakistani population. It would have always been in their interests to undermine the US-backed regime, partly out of general principles that they'd prefer it as their own back yard, but with an additional edge because the USA is significantly more friendly with India than Pakistan: and as we know, Pakistan and India do not get on well at all.
It would have been worse if Trump did it, considering the fact that Trump had released 5,000 Taliban fighters, Pompeo talked to the Taliban, and one of the Taliban that Trump released is set to become the new leader of Afghanistan.
I don't know it would have been worse, but this swing to be friendlier with the Taliban I think reflects the US government as a whole knowing the Taliban would win. Even if too little too late to use as a leverage to hold them back from an offensive, it signalled to the Taliban that the USA is willing to work with them.
The situation is probably a lot more complicated than just 'Taliban won'.
Yes, and I think it's trickier than that in many ways.
The Taliban 20-30 years ago were extremely insular and unworldly in global politics. They are not anymore. They have clearly realised the value of PR and international opinion. They held back outside Kabul until it was clear it was an open door, and they've been in many respects restrained and relatively pleasant - firmly asserting themselves, but keen to display a constructive and practical transition.
Afghanistan has also changed. The allies really have vastly improved Afghanistan in many ways. Virtually everywhere has electricity, mobile phones, media has expanded, never mind higher human development with better health, education, etc. The Taliban now know they can't do what they like without it being likely everyone in the world will find out and harm their cause. If they undo a lot of those advances to run it like the medieval era again, they risk becoming very unpopular and undermining their rule. Thus I would not be surprised if that whilst it will still be shit and much more restrictive (especially for women), it might be significantly more moderate than pre-2001.