I like what this guy named Matthew said Jesus guy's ideas were. Mark and Luke said the same but weren't as good as writers. And another guy named John wrote a fanfic expanding on the original.
And then a guy called Paul (aka Saul) got involved despite never having met the OG crew. And started an expanded universe messing up the canon forever.
I mean, Mark had to go first, so we can give him some slack. And Luke isn't a worse writer, just a bit of a pretentious one. John is definitely the best storyteller, even if he does go...off the rails a bit. And Paul didn't write nearly as much as we give him credit/blame for.
There is that idea yes, that there was an oral tradition for fifty years until the author of Mark came along. It is also possible the Mark Gospel was two lost written accounts that were merged, meaning the oral tradition was much shorter. About 85% of Matthew is found in Mark and most of the 15% are Greek legends. Luke seems to have had Matthew, Josphius, and Mark. John seems to have had the three plus that weird community that might have existed in Jerusalem. I hate using weasel words but really this is all speculation. Not convinced the M source had to have existed, since you can get there without it.
It is worth mentioning that the story about the adulteress in John was in oral form for a long time before written down. So, yes it is possible that this feat happened more than once. Pity that story is even less likely to have happened compared to the rest of the stories, given how nice it is.
I am aware of the two-source hypothesis. My opinion is that those two sources probably derive from oral tradition, and this oral tradition is also the source material that Paul drew from.
I think it is a mixed bag. The teaching about divorce messed up the lives of a lot of people. Then there was that time he told a guy to abandon his dying father to go preach with him. He called a gentile woman a dog. He said most people will be destroyed. Jesus as chronicled by Matthew seems fond of referring to everlasting fire.
To be clear, I am not saying he didn't say anything good. He said don't be a hypocrite. He said help others and pay your taxes. He said feed the hungry and clothe the naked. But that doesn't erase all the bad things the character of Jesus said.