There are actually mainstream Biblical scholars who propose that all 4 gospels were written mostly independently of each other based mainly on oral tradition.
The argument is that Jewish society at the time transmitted information primarily orally, oral tradition was regarded as more accurate than written sources. The argument goes that they were very practiced in transmitting stories without changes and had been doing so for generations. The gospels were all written within living memory of Christ's life. We're not talking about hundreds of years.
There are also Biblical scholars who believe that Luke was based on eyewitness accounts through interviews both written and oral. These Biblical scholars usually are usually also Christian, but in their academic works aren't arguing from a "spiritual position".
My personal opinion, based on reading many sources on this topic, is that it's likely that Luke combined early written accounts from Mark and other sources with witness interviews to produce his gospel.
"Inasmuch as many have undertaken to compile an account of the things accomplished among us, just as they were handed down to us by those who from the beginning were eyewitnesses and servants of the word, it seemed fitting for me as well, having investigated everything carefully from the beginning, to write it out for you in consecutive order, most excellent Theophilus; so that you may know the exact truth about the things you have been taught."
However, again majority of these other scholars and the other theories that you refer to still claim that the synoptic gospels aren't "independently" written, that is, that they weren't written only on "oral traditions." The scholars are aware that arguing only "oral tradition" implies even more "edits" and "inventions" over the time between the events happened and were written, that is, that they'd have to assume that a direct "intervention from God" prevented "the Chinese whispers" effect that would ensue. And that the scholars can't believe because if it were so until the point the texts were written the first time, it would have continued to be even more so afterwards and they know it isn't so.
That's why I've entered the discussion, because I'm really not aware of scholars that would even use your choice of words. You can of course say that you personally don't even have to believe what any of scholars claim, and I wouldn't have reacted were it obvious for me that your argument is based purely on your faith, as in you having a "strong belief in the doctrines of a religion, based on spiritual conviction" rather than using a scientific approach. Again, I'm not aware of scholars that openly argue only from their "spiritual position" who are even accepted as being scholars from the rest of them. Because if "God made it intentionally so with all these errors and inconsistencies" all their efforts are in vain, and there's no point to even discuss.