So, this is an example of the sort of reasoning that I am talking about, though there are actual academic papers better and more directly engaged with the evidence too:
https://carm.org/analysis-pre-pauline-creed-1-corinthians-151-11
I think also you may be using an anachronistic meaning of “eyewitness” account. In these cases, we are talking about statements and facts that can be traced back to people who would have been eyewitnesses. So, for example, even if Matthew and Luke draw on Mark, that does not some how negate them. There is good evidence that they were composed by people who were eyewitnesses themselves or in direct contact with eyewitnesses, independent of Mark, due to what some people have called
“undesigned coincidences.”
https://crossexamined.org/undesigned-scriptural-coincidences-the-ring-of-truth/
What is an undesigned coincidence? An undesigned coincidence (so-named by J.J. Blunt and first discovered by William Paley) occurs when one account of an event leaves out a bit of information which is filled in, often quite incidentally, by a different account, which helps to answer some natural questions raised by the first.
There are two categories of undesigned coincidences pertinent to the New Testament: Internal and External. As the labels suggest, the former concerns details which are filled in by other Biblical (i.e. internal) sources, while the latter concerns details filled in by other extra-Biblical (i.e. external) sources.
I would add that this does not even touch on the equally important and relevant questions of dating of specific manuscripts by various means.
We all should agree that “stuff happened” in First Century Palestine that ended up being important somehow for some reason. There is evidence of many sorts. While a bound bible off a printing press isn’t direct evidence, it is referent to thousands of actual manuscripts, many of which date back to that time, both Scripture and non-Scripture.
One of the more interesting exchanges I had at a LCMS seminary was over evolution and textual criticism. He was a textual critic. We were discussing evolution, and how it progressed. He had a lightbulb go off, fairly quickly, and said (paraphrased, “they way you are talking about nested clades and phylogenies, that is exactly how we think about biblical manuscripts, and that makes a lot of sense to me.” Turns out that textual criticism ends up very similar to phylogenetics. From looking at changes to the text and the script over time, the divergence over time, they can make surprisingly good estimates to date individual copies and source texts, much as we do in phylogenetics.
Of course, I am no expert there and I can’t speak the original language, and I certainly don’t have the nuanced expertise to recognize and place script styles. This figure, however, really was surprising for me,
https://journals.uair.arizona.edu/index.php/radiocarbon/article/viewFile/1537/1541
The black is C14 dating and the grey are dates from “paleographical” analysis. The concordance is and was surprisingly high. Basically, they already knew the dates of the scroll, including picking out the recent forgeries, before C14 dating, and they might even have higher precision. When you think about it, this makes sense, because looking at the text, they can put manuscripts into order with one another, knowing based on the text which one came first, and because there are so many manuscripts from this time, they may be able to get far higher precision that C14.
I could go on here, but the key point is that this is a legitimate and massive field. It also has “terms” of art, where terms are used in ways that makes sense in the domain. One such example is “eyewitness,” which might be best understood as a gloss on a more refined scholarly concept than what our current meaning of eyewitness is (photo on your cell phone?).