There are some elements that I recall from ds2 here. In particular, attitude at school. He started at very nearly 5 and was, tbh, over-confident in the extreme. This was almost entirely the result of attending a very small village school where he managed, a couple of weeks in, to develop a quite ludicrous sense of his own importance based, in the main, around the mutual dislike between him and his reception class teacher. I still remember her coming out to the school gates one afternoon to tell me that he'd been "intolerable". A comment that merely increased his misplaced assumptions.
On checking with the 18 month older ds1, I asked quite what ds2 was doing at school and was told that he'd appointed himself class clown and since he was genuinely amusing, this made him popular with his peers. He'd also discovered that his teacher was keen to send him out of class at the slightest provocation. When sent out, he had a very comfortable morning sitting in the hall while the dinner ladies fussed over "that poor little boy" and fed him home-made biscuits! So as far as ds2 was concerned, only good came from his teacher's attempt at discipline.
The following year we moved house and he started at a very large urban primary school which had far too many pupils on the roll to be allowing one small boy to rule the roost. No special chairs or mats for ds2 any longer, just a quiet but firm expectation that he did what he was told. Which he did and the rest of his school career passed mainly without incident and certainly without him being labelled as disruptive or intolerable.
So I'm wondering whether the OP's ds is also getting the wrong sort of attention at school and this is helping to foster his defiance. I'm certainly not keen on the idea that he has a special mat all to himself because that has to increase the sense of his own importance.
DS2 was never a particularly defiant child at home but he certainly had all the answers. I would allow a certain amount of pleading his case but back chat wasn't encouraged. He did respond well to a final warning system knowing that beyond this he would lose the chance to do something he liked.
Nobody played the drama queen (ironically ds2 went on to get a drama degree!) because it would have simply resulted in hilarity but also I didn't ask anyone to behave well, I expected them to. That's not to say that this was 100% successful on all occasions but I always felt that it was better not to put the idea that there was a choice into the heads of my dcs (or indeed any dcs).
So for what it is worth, I think you need to praise his good behaviour and quietly put a stop to any of the nonsensical repartee. If he is disrespectful then use consistent but simple sanctions that are realistic and immediately applied. Don't let him think he has a choice about behaving well. Somewhere along the line he has picked up on your anxiety and is milking it and while it is easier to suggest than to do, you need to get back some element of leadership.