No, but I married well. My DW has significant experience as a caregiver for children other than our own, some of whom have presented to their parents in much the same way as you describe. And after a bit of time with her, most of the willful behavior disappears... only to reappear the instant the child's primary caregiver arrives on scene. For example, my niece used to stay with us a lot, and she was a "flight risk" who would run out into the parking lot when she was your son's age. DW put a stop to it. Anytime we went anywhere with my niece and her parents, my niece would run again.
but are probably not the firmest or most consistent with our boundaries.
That would appear to me to be the issue. My DW and I are guided by this simple principle: Choose your battles carefully, but win all the ones you choose.
And above all, this principle: consider the child's perspective, and what you're teaching to that child with each seemingly-innocuous interaction.
One thing to understand is that the world is a confusing place to a 2.5yo, and they have to test using cause and effect in order to make sense of things. When results are predictable, the child gets a sense of order, and it can be quite calming and comforting. When results are unpredictable, the child must continue to test and gain more data. The child may also become frustrated, confused, and frightened when the results don't match expectations.
Consistent boundaries and consistent responses from you are key.
It's also hard because he is amazing and social and sensitive. and the temptation is not to crush or stamp any of that out. How do you not crush his spirit and his abilities and his opinions and will, yet still be firm, authoritative parents.
First, I would advise you to abandon the notion that exercising authority is associated with crushing spirit. The arbitrary and capricious exercise of authority can do that. The responsible use of authority, on the other hand, is yours because he's a child, and it's the best way you can keep him safe, socialize him appropriately, and nurture him in ways that guide him towards becoming a responsible, functional, capable adult.
The setting of boundaries still allows for free-spirited exploration, because we allow our child to, within the established boundaries, go nuts. We're also careful to never create a rule or boundary without a very good reason for it. We should always be able to answer a question of "Why not?", even if we wouldn't actually answer it to her based on age-appropriateness.
Second, I would advise you that any change you make in your parenting approach is going to exacerbate problems in the short term, because your DS has become used to certain responses from you, and any change you make will create more confusion, leading to even more acting out. Only when you've established a pattern of consistent responses to his behavior will he start to settle down, and that takes time and commitment.
[My DW and I remain convinced that this is the major reason why "terrible twos" is a thing, because most parents impose very few rules and consequences on their children until that time, and then the child experiences what seems to them to be a complete upending of what they understand about the world and their place in it, based on a complete and unexplained change in the nature of their interactions with their parents (who make up most of their world).]
Third, I would advise you to start talking with your DH over every seemingly-insignificant interaction with your son, so you can establish a uniformity in your boundaries and responses between the two of you.
Finally, I would advise you to spend quality time with your DS in child-led play every day. It strengthens the bond that makes him want to naturally please you and earn your approval. It provides you with natural opportunities and teachable moments to help him work on his boundaries and socialization. You've indicated that you feel like you can't handle him, and he's in school partly to get a break from him... if he starts to get the sense that you're avoiding him, that will only increase his insecurity, and he'll act out appropriately.
