Maybe you should watch this video.
I don’t think you have a concept of how many manuscripts 5400 (more like 5700 at the moment) is. We don’t have this many ancient manuscripts for any other ancient document. 5700 is a huge number of fragments. You can debate whether or not what the NT says is true, but you can no longer debate whether or not we can determine what it originally said. Scholars have pretty much stopped disputing it. With exception of a few phrases here and there, we have by far enough manuscripts to figure out what was originally written.
As for authorship of the Gospels, here’s another video if you wanna watch it.
Whether or not some of the gospel writers were eye-witnesses themselves, we have very good reason to think they knew a lot of eyewitnesses and they were written while eyewitnesses were still around. Of course there were no eyewitnesses during the official canonisation under Constantine. That’s not what I said. I said that the authenticity of the canonical gospels was well known long before Constantine was born. There were some NT books in dispute during his lifetime, like Jude and Revelation, but none of them were the Gospels. The canonisation was a long process that didn’t just suddenly happen one day. The process began pretty much when the NT was written. we have documentation of people’s reception of the Gospels all throughout the first few centuries and we can roughly see how they became canonical. They were not suddenly arbitrarily decided upon in the fourth century.
The canonical gospels, out of all the known gospels including the gnostic and apocryphal ones, were written by far the earliest and by people most closely associated with Jesus himself. My point is, that’s why we have four Gospels.