I think ultimatums are generally really pointless TBH. I know how easy it is to get sucked into issuing them, usually because someone just isn't understanding something important or because they are continuing to do something, saying they don't mean it then doing it again....
I got into a similar cycle with my (horrendous) x where he would keep treating me like shit, I'd get desperate and break up with him and he would desperately pressure me by promising the world, I'd feel trapped into staying with him, he'd continue doing the thing, I'd get desperate and break up with him etc
I think if you issue an ultimatum it must be because you mean it, because you really need it for you to feel able to leave rather than because you expect them to heed it and you must follow through on it.
The nature of it being an ultimatum must be secondary to the fact you are expressing that you cannot continue in the same vein IYSWIM? The point has to be if x continues I will y, not to coerce the person into stopping x....
Also, I think where addiction is concerned ultimatums often just provide a focus for the addict to continue indulging their addiction - 'you saying x makes me feel stressed so I drink, if you stopped stressing me I would drink less' etc etc.
For an addict that hasn't acknowledged they are an addict what you are doing when you challenge them to reduce their using is basically, as they see it, saying 'this thing you rely on to cope? I demand that you stop doing it'. This causes panic.
Addicts are often addicts because they haven't developed skills that they need to cope with life. The addiction is a maladaptive coping mechanism and without it they feel they will not be able to cope in order to give it up they need to know it is a maladaptive coping mechanism and develop healthy tools for coping to replace it (separate from the physical addiction).
I think you would do well to focus entirely on yourself and any DC. Do you want this life? Do you want him in your life? Him how he is not how you would like him to be...