There are always two sides to the story. OP by opening your marital woes to outsiders you’re going to get a lot of unhelpful advice and judgment. I hope you’re okay with reading our opinions. Bear in mind, this is your marriage, it should be protected as much as possible if you still love each other, if you don’t care then why continue?
From what you’re saying there’s a sense that your husband feels frustrated and resentful too, I don’t think he would have suggested separation casually unless there was some sort of frustration/resentment against you.
It also sounds like he has been stressed especially since he picked up smoking again. It might be that he needs to cut back on his hours at work. Why work so hard for money when you’re both unhappy?
He’s probably at the gym because he’s running away. Maybe avoiding you as you’ve said because you said he says you “complain all the time”.
Do you think he feels appreciated and valued as a member of the family? Do you criticise him too much and not give him enough affirmation and love? Do you tend to control him a lot? A lot of women do and that can stifle men a lot. They’re men, they need to feel in charge, even if you’re really in charge. They need to feel alpha. That’s their biology.
We all know that most men are inherently lazy. There are ways to “train” them, you have to be clever about it and bossing him/telling him constantly that he’s not doing enough is not the way to do it. You could play chicken and do less or even nothing, and he will learn to not take you for granted. You could even go away to your parents for a few weeks, absence makes the heart fonder always. When he does something right/helps, do you thank him for it? Do you show your gratitude?
It doesn’t sound like he’s that awful given that he steps in at the weekends and also stepped up his game to help with your daughter when you were off sick for a week? Again because I only have your perspective, I can’t know for sure what the truth is.
Lots of couples fight because they take each other for granted, have unmet expectations of each other and over time this builds resentment because they don’t want to talk about it.
Obviously you both love each other that’s why you’re building a family together what with number two on the way. It sounds like you both need more support, why don’t you both allocate some time to talk things through? Be honest and open with each other.
If money is not an issue for you both (I mean smoking and gymming cost money), can you outsource help? Get a cleaner? Pay for additional childcare?
I feel like it would be really helpful if you can reflect on how you can improve the situation. Not what he can do but what you can do. End the blame game and victimising yourself and take some responsibility because this is your marriage OP. Protect it or lose it.
Good luck with everything. I hope you get the outcome you desire x