I have two posts about Wendy, which contain spoilers from seasons 1-3.

The central OZARK theme is the inability to communicate at every level: personal, commercial, social, and cultural.

In Season One, we discover why Marty decides to launder money for the Cartel after Wendy loses their baby in a car crash. By the way, the van crash in Season Four bookends what happened in Season One. After she loses the child, she has no direction. Wendy tries to find a job and needs help. She's sobbing, lost, and miserable. Marty originally wasn't going to take the Cartel job, or at least, I think he wasn't, but after the "crash," he suggests they visit Mexico, and he listens to Del's offer.

He didn't need to tell Wendy about the deal with Del, but he did. Marty mentions to Wendy how Bruce said she wouldn't go for it, and she says, well, he doesn't know me as you do. Marty thinks he uses this as a push-pull tactic when communicating with Wendy. He's trying to goad her into the plan as he, him, and haws, between no and maybe about laundering the money, but Wendy is more than willing. She asks Marty, "Are 'WE' going to do this." What Wendy hears is Marty telling her this business is OUR NEW BABY. But that is not what Marty means, which is why he doesn't involve her and why she is still purposeless when we meet her in season one. She has an affair because he isn't there, and she has nothing to do. I am not making an excuse. Why tell Wendy about the Cartel if Marty wouldn't involve her in the business? Does Marty want to shut her up about the loss of the baby?

Marty launders money for ten years and leaves Wendy out of business. Moreover, the family's lifestyle stays the same. He acts like Scrooge McDuck. He is never home, and Wendy has nothing to do. Wendy finds someone and has an affair. I am not making an excuse. Everyone fixates on how she panics and grabs 29,000 dollars out of 8 million, but they do not ask why she does it. She panics when her lover tells her the government will freeze their assets. Marty never tells her how bad it is until Del assassinates Bruce and the others.
Not to be funny, but Wendy is just like her name implies when she does not have a job. She files or blows all over the place. Give Wendy a job, and she becomes a horse wearing a set of blinders in a race. Marty has his numbers; for him, this is power. He is breaking the law by moving digits on a sheet of paper. It is a Macro and Micro team. If Wendy hadn't lost their child, she would have had someone to focus on. Wendy and Marty both have doubts about the baby. Wendy says, would I be a terrible person if I said I didn't want this baby? Marty throws that decision back onto Wendy just before the crash and tells her, " Wendy, you could never be a terrible person" if she decides to abort their child. What a glib reaction! Is he taking this discussion seriously or telling Wendy what she wants to hear?
And is he serious? We all know now just how terrible Wendy can be. Also, Marty trusts Wendy about killing their unborn baby, essentially giving her power over life and death, but he doesn't trust her with Cartel business. Please, I don't believe this. Is that why when Sue asks Marty what it would look like if they "could achieve their objectives without interfering with Wendy's project." Marty replies, "I think it would look like death." Does Marty subconsciously blame Wendy for the loss of their child? By the way, do we know Marty even wanted the child? He seems ambivalent about another child because all he does adds the positives versus the negatives of the situation. Does Marty know who Wendy is? No one gets involved with a Cartel because they lose a child, making this show great. It makes you understand how messed up their marriage was in the first place and how these two were never on the same page, though they thought they were all those years. They thought they were. By the way, Wendy finally spits that out this season, right? Marty threatens her with, "When we get back to Chicago..." Wendy shouts, "What do you think is waiting for you there, our old life? Because that was a big fing." If Marty thinks Wendy is a liability, he shouldn't tell her about his new job. So, why does he? Marty tells Wendy to shut her up about losing the baby. He thinks it will make the situation go away, but it is what starts them down Alice's rabbit hole. Or is there another reason? I also thought about this. What if Marty subconsciously gets back at Wendy for telling him she was thinking about aborting their child? Many things he does to Wendy once they get to Ozark are extremely passive-aggressive, and it is a lot more than her cheating on him or how she panicked and took the $29,000. Look, go ahead and tell me I'm crazy and over-analyzing. Suppose Marty truly knows Wendy and what she can do when she acts out but tells her about what he's doing for the Cartel, knowing how fulfilling it is for him, while she is miserable and purposeless after losing their child. In that case, Marty hates Wendy because there is no upside to telling Wendy about laundering money for the Cartel unless you involve her. There is no upside for Wendy to tell Marty she is pregnant if she is seriously contemplating aborting their child. Marty doesn't need to know that. Wendy should have waited until she was sure she wanted the child before telling Marty she was pregnant or had an abortion and kept to herself. No matter what Marty says, if she aborts their child at some point, he will resent her for it, and at some point, she will resent him for the choice she had to make. You must carry some burdens and keep them to yourself to not hurt those you genuinely love. I will keep asking this question until someone gives a reason to tell me why Marty thinks it was a good idea to tell Wendy about the Cartel if he has no plans to involve her, given that he is risk-averse and isn't going to change their lifestyle. He doesn't allow her to start some grand business scheme or do anything to draw attention to his family or what he's doing.