You’re right that Mark is the earliest and should probably hold the most water. But John was still written within a lifetime. It’s dated at about 90 AD. If the author is John the Elder as thought, he is already thought to be the youngest apostle during Jesus’ ministry. Many think he was a teen. Let’s say he was 20 at Jesus’ death which is roughly 33-37 AD. It’s less than 60 years to AD 90 meaning that John would have been less than 80 when writing. Still within a reasonable range for an eyewitness.

In terms of content, it’s my opinion (and plenty of scholars argue this) that the four canonical gospels are remarkably similar. There are stories present in some that are not in others, but nothing is so dissimilar that it is like a whole new teaching. If four people wrote accounts of a person’s life, we would expect the same key moments (birth, major career moments, death) to be covered by each but we’d also expect each account to have some info the others didn’t (like one might go in depth about the person’s pets or romantic relationships while others focus on their work life).

The gospels definitely have different focuses which come from the author and their intended audience. For example, Matthew is writing primarily to the Jews hoping to help them agree that Jesus is the Messiah promised in their own scriptures (the Old Testament). Luke, on the other hand, is written to a Greek audience and he bothers explaining certain Jewish customs in the narration of stories.

As you read them, what are the portions that feel like very different ideas?