It could be so many things. They have raging hormones. They have social issues at school where they will be ostracized if they snap back at someone, so they do it at home where it's "safe". They're starting to separate more from parents in preparation for leaving the nest. Sometimes that actually terrifies them. They do get to a stage where they think they know everything, and they aren't interested in safety or cautionary comments from parents. Remember that their bodies are pretty mature but their brains done reach maturity until age 25 or so. They have no concept of how hard it is to work and pay bills and provide the opportunities and so on. Sometimes you get some dream kids (like Suz mentions) but the moody/angry thing is probably more common. It just sounds like you have a tougher case.
There may also be unique issues in your son's case - maybe he has stepfather issues, father issues. Maybe he thinks that his therapist mom is analyzing him vs. talking to him like a parent? Doesn't mean you are, just means maybe he misunderstands. Maybe he's got social problems at school. Maybe he's abusing chemical substances in some way - and you should open your eyes to that possibility - grades and sports aren't proof that he isn't. Maybe he's been abused or bullied by someone - a peer, a teacher, a coach?
It's interesting to me that you are a therapist yet use the term "wasting thousands on therapy." Can you say more about that? Why was it a waste? Because it wasn't a good therapist? Because it didn't work? Maybe the problem wasn't discovered, maybe your son didn't cooperate, maybe he (or you) think that you should be able to handle everything just because you are a therapist yourself? That really concerns me that perhaps you haven't been able to find the right match for a therapist for both of you together, perhaps with his father or stepfather as well.
There's a book called "Get out of my life, but first can you drive me and Cheryl to the mall?" or something like that. It might be helpful to give you support through the love/hate from the typical tween/teen.
I don't think you need to buy shoes for someone who is disrespectful if it's just a luxury purchase, not a true need. I think engaging without the anger he is showing is a wise choice - don't snap back at him, but do as Rosebud suggests below, saying you're not rewarding the disrespect. Small, immediate, measurable consequences are much better than long term, without-end punishments that just push a kid away and feed into his "Nobody likes me" misery. Letting a kid this age have some sort of larger allowance (out of which he can buy his own clothes, pay for his pizza/movies, and generally learn to budget) isn't a bad idea. When they run out of money 2 weeks into the month, they start to appreciate how much things cost and what all you are paying for.
My guess is, when he turns 16 or 16.5 and wants a learner's permit and driver's ed, you'll be able to say "I'm not comfortable giving the keys to a $25,000 car to someone who's a hothead or who will disrespect traffic laws and the police."
I'd also say that you need to check for other causes of deeper concern - you should monitor his internet use, check his browser history, look at his cell phone, and educate yourself to the types of apps kids use to hide their true actions from parents. There are kids with serious psychiatric issues who engage in all kinds of harmful behavior (to themselves or others), and parents who didn't consider that often have devastating regret.
But otherwise I think deescalating is a good thing - but you say this is hell but not specifically how bad it is. As always, be willing to seek better outside help. If this were an illness and the first doctor didn't fix it, you'd find someone else. Getting support for yourself as well as your kid is a worthwhile effort, and not a "waste" of money.