Pre-Christian paganism ended up becoming the basis of a lot of Christianity and seems to have been greatly absorbed by it. However, it seems mostly anything today that I see that is Pagan is just new age mysticism and doesn't appear to have any roots to traditions and practices of the past. Is that a fair assumption?
There are two primary "types" of paganism these days, modern (also called neo) paganism, and historical paganism. These are by no means the only categories, and there are a lot of subcategories, discussion, etc. People practicing either will typically refer to themselves as pagan, so it can be difficult to know which kind you are looking into.
Historical paganism tries to be truest to pre-Christian forms, working from historical information including but not limited to written works, art, and oral tradition. Often some interpretation is necessary as there can be conflicting information, or information provided by a biassed source (for example, documents written by Christian observers). The field is not without controversy.
Modern/ contemporary/ neo paganism is less focussed on being true to the details of history, and more on the "spirit" of the practices. Some are syncretic in nature, borrowing from various practices, while others are simply entirely new practices.
So to answer your question, it depends. But consider the people who practice historical paganism as their religion. It is of primary interest to them to pursue their traditions accurately, though often they have to take the word of others that the research have been conducted with integrity.
Pre-Christian paganism ended up becoming the basis of a lot of Christianity
Statement rather than question - can you justify that? Yes, there are some peripheral bits that were inherited (e.g. St Bridget of Ireland was synchretised with a pagan goddess of the same name). But what core beliefs are you thinking of?
It's difficult to say with any certainty what paganism was like while it was practiced. Most documented information about paganism was written in post-pagan Christendom or by contemporary observers, as very little writing survives from pagan societies. Runes were only used rarely to write sentences, and by the time adoption of the Latin alphabet was present, many people had also adopted Latin religion.
Can you cite some specific aspects of paganism with Christianity that you are referring too?