In all honesty it really worries me that he's only going about the smoking. What's the GP going to do, prescribe a few patches? The GP needs to know exactly what's going on. All of it.
Cannabis can have a very detrimental affect on mental health. I appreciate he has been self medicating after a very difficult time, but many, many people go through difficult times, losses, serious health problems etc and do not become regular drug users. He is not coping with what he's been through he's just blocking it out.
You say you're sure he's suffering from depression but long term cannabis use can also present in this way. Susceptible people can also develop paranoia, hallucinations, and even end up with schizophrenia.
You have said that you believe that his mental health is at the root of all of it, but there is a good chance that it's actually the cannabis at the root of it. It sounds like he is a long term user, even if before it was more "social". Your point about sorting out all of him, mind, body, soul, etc is good, there are probably many areas of his life he needs to work on, but i truly believe that he needs to stop the drug use and at least reduce the alcohol, before he can do any of the other things.
Incidentally I sincerely hope he doesn't drive whilst using drugs so heavily.
He needs experienced, specialist help to deal with his drug use and mental health. Stopping smoking is not even going to begin to address this. You mention he has had a serious health issue, I understand that you may not want to share what, but you need to consider if this is something that will be affected in any way by regular drug use. Is he on any regular medication for this? There could be serious effects around cannabis use and any prescribed drugs.
Ultimately your DD is at the centre of all this. He has told you he doesn't want to stop using illegal substances. What about when they learn about drugs at school? Believe me, they start very early these days (albeit in very basic terms) in KS1. Won't she put 2 and 2 together. She will surely begin to realise what he is up to in the garden, and it will be more and more difficult to hide from her.
Or what about if she goes round to a friend's house to play? The Mum goes out into the garden to have a quick cigarette, and your DD pipes up "Oh, my Daddy smokes, but his don't look like that and they smell funny".
In your last post you say that if things don't improve you may have to issue and ultimatum. I sadly believe that is your only course of action. This is a man who will willingly spend at least £3640 a year on his drug habit. Out of choice. Knowing that as a family money is tight. His priorities are so very, very skewed that I think it's going to take an ultimatum from you to make any difference. Then it is his choice to make. The help is out there for him.