How early would the first canonical Gospel need to be to (virtually) eliminate concerns over inaccurate memories?

Some scholars have challenged the accuracy of the Gospels by claiming that eyewitness memories and oral tradition methods could not ensure perfect retention and transmission of the extensive teachings and encounters of Jesus, if several decades indeed transpired between Jesus's death and the publication of the first Gospel , and perhaps half a century or more transpired before the publication of the final Gospel, as is the common Gospel origins theory. For example, in Robert McIver's “Collective Memory and the Reliability of the Gospel Traditions,” he claims that “much can happen to traditions that are preserved in human memories for this length of time.”1 He then "focuses much of his attention on the frailties of memory, in the areas of transience, suggestibility, and hindsight bias. These can drive variations in details and also drive falsehoods, not only when eyewitness testimony is presented in court, but also when past events are remembered in general."2 Therefore, I need to ask, how early would the first Gospel need to be to avoid this challenge to Gospel accuracy due to memory loss and transmission errors? 1 Robert K. McIver, Memory, Jesus, and the Synoptic Gospels (Atlanta, GA: Society of Biblical Literature, 2011), 1. 2 Daniel B. Moore, A Trustworthy Gospel: Arguments for an Early Date for Matthew’s Gospel (Eugene, OR: Wipf and Stock, 2024), 93; McIver, Memory, 10–11.