He is showing you his move though, it may appear to be inaction, but it’s a clear choice. I think that people are encouraging the ultimatum scenario as a way of empowering you, and allowing you to take back some control. There’s also a possibility ( albeit tiny ) that he might finally realise what’s at risk, if we assume he hasn’t until now.
You are correct in that your marriage as it was is over, whatever happens. I think through time you’ll realise it wasn’t all as it has seemed, whether you are apart or together. That’s not a bad thing necessarily.
So far, and understandably you have been focused on how you two, as a unit get through this. I strongly encourage you to start to try to change that mindset. It doesn’t mean you are giving up on any possibility of reconciliation. It does mean you can focus on you, on your DC, and your needs, not his no matter how much you want to help him still. It’s not easy to do this, but it’s crucial to moving forward. At this moment in time you are not that unit.
If nothing else is clear, what is surely is that, even without the relationship, his selfish prioritising of himself, and his hobby has negatively impacted you and your DC, whether or not you realised it. It is unlikely to be sustainable at any level going forward.