18 months is still a massively demanding age for a child op!
It's so difficult because I think dad's typically have got used to this one person, their wife, meeting all their emotional and physical needs - and then when wife has to redirect her energy to a new little person, the dad feels left out in the cold. It's an unfortunate part of the human reproductive cycle...
She sounds exhausted and depressed, and like she's trying to carve out what little space she can in her life for a feeling that no one is demanding anything from her. I've been there. I have a VERY high sex drive, but even I, at that stage of DCs life, was absolutely wrung out.
In my case, my ex was panicked by the reduction in attention from me and became more and more anxious and eventually controlling/ possessive / suspicious. It was really painful.
At the root of it all, op, is a difficult truth that men usually don't anticipate the fact that when children arrive, they suck most of the energy out of the partners, and for men who generally grow to expect women to attend to them, that can be hard to take. I don't think your wife has much energy to give you at present. It's not your fault or hers, it just is.
My advice is, pursue your own interests as much as you can, but involve DC in them. Take him for long walks, get him out in nature, really focus on him. Invest emotional energy in him and reap the rewards of that. Take accountability for him, his emotional development, his social life. Make friends with other families / dads, plan his wardrobe, shop for his bits and pieces, plan parties - take on some wife work/ mumwork. Not just the tasks - the headspace.
A corollary of that though is, make it easy for your wife to pursue her own interests as well. Give her space to remember who she is and recover a bit of the energy she is ceaselessly giving to your DC. Encourage her friendships, help her get out of the house.
The key is, don't focus on encouraging her to be intimate with you. I know that's hard to hear and maybe counterintuitive. But in the nicest way, be aware that men usually have no idea how much they demand, emotionally, of women. One of the reasons my ex and I had to split is because he became obsessed with how I didn't "seem" happy enough, didn't "seem" to want to have sex etc... Basically what he was saying was, ffs why can't you perform the role of a happy wife so that I can feel good about myself? Why do you have to have feelings all the time... It was exhausting and, emotionally, backbreaking work to keep myself "seeming" the way he required me to be, in order to boost his ego and make him feel safe in the relationship. All I wanted was to be seen and heard for who I really was during a massively challenging time in my life. I had no energy to perform for him. And he hated me for it.
Getting space to be with friends, go to nice exercise classes, have a coffee in a pretty setting by myself, with no curfew and without a million texts asking where obvious item XYZ was, would have helped so.much.
Also understand this... Don't do things for your wife in order to get a result. She is not going to give you a result, and she will know you're only being good to her because you want something back from her. She's going to be exhausted ans drained for years yet. Your goal here is just to support her. Until she recovers. She can't perform for you right now. If you don't support her now, when/if she does recover, she will know you were not on her side when she was low. And that will probably permanently destabilize the relationship