But some of those "biographical accounts" contradict each other in important ways.
Eyewitness testimony from people who observe the same events is usually inconsistent or contradictory in some way. If a group of people all independently repeat the exact same serious of events, that doesn't mean that is how it happened, it means they got together to get their story straight.
There are also many subtle details which support the validity of the gospels. In one gospel
but the scholarly consensus is that they are anonymous, and those seem like speculation
They were originally anonymous, which was common back then. But that doesn't mean we can't figure out details about who wrote them.
If the authors of the gospels were eye witnesses, why did they wait decades to write them?
They had a lot going on dealing with the aftermath of Jesus's death and resurrection, trying to maintain the movement, establish more churches, spread the good news, and trying to not get martyred. The earliest Christians in the years after it all happened didn't need a document telling them what happened because they were there when it happened.
Then eventually they realized that they needed to start writing it down to teach to new converts who weren't eyewitnesses, and as the apostles started dying off.
They must be copies of older writings, which we don't have.
Yes, manuscripts typically did not last for very long, and everything is a copy of a copy of a copy. This is true for every historical document from that period of time.
But we do have stuff, like the P52 manuscript fragment of John which is dated to around 100AD and found in Egypt, meaning that the Gospel was already in wide circulation by that point.
not one person who supposedly saw those miracles has writings that survives
The only Gospel that we know wasn't written by an eyewitness was Luke, because the author literally says he wasn't an eyewitness, but used the best eyewitness testimony and other sources.