I was in the exact same positions as you 8 years ago. I had a newborn and a partner who smoked weed all day. No help with the baby whatsoever - living to different schedules because he would stay up all night smoking weed and be unable to get up the next day (he did hold down a well paid job because he's smart and hours are flexible in his industry).
I probably would have saved myself a LOT of effort if I had just had the confidence to leave tbh. I didn't, we are actually together now and have two children. We both work full time and with regard to household stuff and childcare, I do around 65 percent of it, him around 35 percent. It is far from ideal, I still harbour a LOT of resentment and do more than my fair share of the load.
It has taken a hell of a lot of nagging, cajoling, arguing and negotiating to get us to a position where he takes responsibility for anything. He basically had no interest in his own children until they were around 2 years old and he could have a conversation with them.
Things actually came to a head when my eldest was 2 and I was doing everything, I realised that I would have to go back to work full time for 2 reasons: 1) so he was forced to take some responsbility for childcare, pickups and housework and 2) so I had some freedom and space (financially, physically and mentally). Things improved massively after this. I felt more able to assert myself and my needs and I think he actually has more appreciation and respect for me.
Still, our relationship is far from ideal. There is a lot of resentment from the past, I still consider leaving quite often and I have to constantly keep in his case or we slide back. I get sick of being the only full grown adult in this relationship and never getting to be the fun one! I feel aged before my time and worn down by it all.
On the other hand, we have 2 lovely(ish) children, I have a better career than I would have if I didn't have the pressure to try and gain some freedom and my assertiveness, negotiating and general communication kills have been vastly improved by being put to the test constantly by him. I think he is also a better man than he would be if I had just left in the begining.
I'm just giving you my experience, both sides - good and bad. I hope this gives you some insight to what things would be like, whether you decide to stay or leave. How much personal growth do you think your partner is capable of? Even if he is capable of stepping up eventually, do you want to put all this time and mental energy into changing him? How much of yourself will you give up in the process?
The thing with partners is that we pick them based on who we have fun with, who understands us, who makes us feel alive. These things don't necessarily translate into who will be a good father to your children and who will be reliable! Monogamy sucks.