Really struggling to deal with this

[SuziSecondLaw]

My 14 year old ds has always been fairly difficult, but has gotten worse in recent years.

He is absolutely lovely 50% of the time, genuinely nice, big hearted human. Until you tell him he can't do anything, and then it's like he's an entirely different person. He's absolutely vile. His dad, who I divorced 10 years ago, my dp, teachers etc all agree with this. One teacher said it's like he becomes a demon.. I couldn't argue with that.
So anyway, then obviously punishment needs to happen, which is either grounded or tech being removed etc and then he starts threatening suicide..! Which terrifies me, frankly.

He is also a trans boy, not sure if relevant but he constantly acts like he's such a victim with a terrible life. Which is not true. I disagree with the whole trans thing personally (5 of his classmates are trans...I mean, there's no way). But regardless, I have been 100% supportive in person, bought a binder, even changed him name legally a few weeks back!

I just don't know what to do. Nothing seems to help, nothing works. How do you punish a kid that threatens suicide??

Teens are massively demanding and yet sometimes hard to handle.
Punishments just don't really work at this age so I would pretty much put that to one side.
As posters have already said, trans identifying children are in high proportions in the neuro diverse. Be wary of rushing to medically affirm as this may not, in the longer term, actually be the root cause of what is happening here.
Continue to be loving and supportive.
Maybe agree with them what you expect with regard to chores/helping but don't fight over it.

I'm 99% sure he says it to hurt me. But that 1% of doubt is enough to be terrifying.

Punishment as in, grounding just walking out when I've stated that they're not to go out until coursework is complete?
What do we do instead then? Just say 'oh well do as you please'? I don't understand this..
Schools, online advice, my own just intuition or whatever thinks that there has to be consequences for going against rules etc. Am I wrong?

I understand, but teenage brains are not very responsive to punishment.
To some extent you have to allow for natural consequences. If they don't finish their work, it's their responsibility. Not yours.
You can say, once you finish your work (insert the treat/positive consequence of choice) here.
If you have an oppositional teen they will just push back to rules that seem punitive.

It's difficult though when I get the school pushing for work to be completed.
He's not very money oriented or anything like that.. So not sure on a positive consequence to offer? I can't say 'if you finish this you're allowed out' because that's the same as saying 'if you don't finish this, you're not allowed out' 🤦🏻‍♀️

Plus, what does happen when he's extremely rude and walking out the house, slamming the door, me not knowing where he is, etc? Do I really just do nothing? Just say 'well if that doesn't happen again today you can have £5?' I just can't believe that's good advice?

Sorry if I'm sounding awkward, I genuinely can't see how this would work, but I'd try anything!!

I should also mention, my dp, my ex dp (both longterm!), the school, and dc's dad all think I'm way way too lenient with him.. So I'm hardly big on punishment!

Suzi, that is all very easy for them to say (that you are lenient), and must be so frustrating for you.
Your calculation that this one DC receives more of your time and attention than any if your other family and friends put together speaks volumes (I've also got one like this), and is indicative of an issue which goes so much deeper than whether you are 'lenient' or not; I bet this dynamic has not emerged overnight, but is where you find yourself after years of having had to respond to your DC's needs in a very deliberate, specific and intuitive way in order to get those needs met and ensure your DC remains regulated?
Some young people have very challenging behaviour and very little sense of the impact of consequences or self-preservation in conflict situations; it looks as if they really don't care. Simple negotiation becomes high stakes confrontation in an instant, and they can not be relied upon to respond to the kind of 'regular' discipline or consequences most children and young people do. I wish people would think and empathise before they remark on a parent's percieved leniency.
I'm a primary teacher with experience of youth work, and have no compunction about being firm with my own DC (like I am with children I work with), standing by my expectations and setting clear boundaries. All good. Apart from with one of my DC who has never quite been able to play ball in that way and to whom the usual perimeters of expected behaviour when in a situation of a percieved conflict of interest appears to matter not one jot.
It's hard and I really feel for you.

My most time consuming child is thankfully our youngest! Even she comments that we are much more lenient than her friends parents. But, we also have very firm boundaries around how we treat each other. I'm not bothered by her room being tidy or doing chores, but I expect to be spoken to politely and for co-operation at all times.
So in that respect I don't punish or have consequences, but I do expect respectful behaviour and in return I give the same to her She knows that I have her back and will move heaven and earth to keep her safe.
DD is my 3rd teen in the house, I have 2 older SC too. Same rules for all growing up.
I don't micromanage homework but I will help them have a good environment to work in, help regulate them when they get stressed (take a break, etc) and be pleased for them when they do well.
I genuinely think there is a difference between if you finish this you're allowed out' and 'if you don't finish this, you're not allowed out' because one is framed as a positive interaction and the other is a threat. Same message, different emphasis.
I would probably, knowing my child was struggling to focus, offer some kind of incentive, or time limit for work to minimise the melt down - say work for 20 minutes and take a break/go for a walk/come out for a drink with me.
But - you know your children and I know mine. All behaviour is communication though and I can't help but think your oppositional angry teen is trying to find the end of your patience and you just have to keep finding more.
It will pass and I wish you both well.

It's very hard, I feel as though everything I do is wrong. My other 2 dc are absolutely fine. I've brought them all up with the ideal of 'praise the good, ignore the bad' but it is so much easier said than done when one child is constantly pushing boundaries.