1) Richard Dawkins is a biologist, not a psychologist.
2) Why would you assume I have not read The God Delusion? I have. It puts forwards a lot of good arguments, but like most things by Richard Dawkins it is very self satisfied, very self congratulatory and you can see that he helps himself to feel superior by denigrating religious people. This is a common theme with RD’s arguments.
3) I did not cherry pick definitions at all, I presented three different definitions of delusion for consideration, two from a commonly used dictionary in which I addressed all definitions presented by the dictionary, and a psychological definition of delusion as presented in a peer reviewed paper specifically on delusion.
Even if we ignore that and just use the definition you present, your definition is itself evidence for my argument. You actually present two separate definitions, in the first you may note the word “idiosyncratic”. Idiosyncratic beliefs are ones that are peculiar or unusual. Religious belief is in no way peculiar or unusual. In fact in many places in the world it is more unusual to be an atheist, as such if you are going to go by this definition to describe a delusional person an argument could be made to suggest that atheists are more delusional than religious people. It also goes on to specify that this is a characteristic of mental illness. Religion is not a mental illness.
The second definition you provide characterizes delusional to mean nothing more than having faulty judgement, and presents it as a synonym for being mistaken. In this case it is essentially meaningless, and literally anyone can reasonably be called delusional. Everyone is mistaken about something. It is impossible to be correct about everything. If this is your justification for calling religious people delusional then you are also delusional, as am I, as is every other person you have ever met.
4) Religion can be a symptom of delusion, but delusions are by nature immutable. They don’t change to account for the changing understanding of the person who believes the delusion. This is not true for most religious belief, which notably changes depending on the individuals understanding of their own religion. Which can change quickly depending on circumstances. In fact one of the most notable aspects of religious belief is how easily they can adapt to circumstances.
There are times when this is not true, sometimes religion is a symptom of delusion, but this is only true when the delusion is a feature of a medical, neurological or mental disorder. I really think you would benefit from reading the paper I gave a link to on delusion, it is specifically about understanding delusion and is titled “Understanding Delusion”. It goes into detail to specify the differences between a delusion and an overvalued idea, which is what I think you may be confusing the term with. You can find it here: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3016695/