A close friend of mine has legal custody of her sister's daughter; due to similar issues. Psychosis before and during pregnancy. In their case, the Dad refused to live apart from the Mum, so Social Services approached other members of their family. Their parents were considered too old and unwell, but my friend has two kids already and was asked if she'd consider looking after the baby. They all knew this was going to happen from around 22 weeks, I believe.
My friend was called when her sister was in labour and went to be with them as birthing partner. Her sister was also on Aripiprazole, although the baby's levels tested well quite soon after birth, so it ended up being the court paperwork that took the time. They had two days together in hospital, with Mum and Baby supervised by a Social Worker and my friend visiting a couple of times a day. I believe she did the initial breastfeed, but didn't carry on, due to the risk of it feeling like "something else" the baby had lost alongside Mum. I think that's probably quite a personal call. It's hard work to express enough for a baby, especially if the baby isn't near you, and you'll have a long road ahead if that's something you choose to do. It could also trigger more symptoms, if you're sensitive to hormonal changes.
When baby was 6 months old, Dad was offered the chance to be assessed in a unit caring for the baby, which he took - but left after a day as he found it stressful. That pretty much bought to an end any legal challenges around custody, so my friend became legal "Mum" at that point. Little Girl has always been very aware of who her actual mum is, there are photos, but there isn't direct contact between her and either of her parents. She does meet up with her sibling, but she's cared for by her Dad, who is a different man if that makes sense. She calls my friend Mum but seems absolutely fine with having two, and she's a really happy, well-adjusted little girl.
I'll be honest that her sister has ups and downs, it's a horrible situation because it's so traumatic, which doesn't then help the psychosis. I know she hoped for more contact, and to have her daughter back with her one day. I believe she was approached about measures she could take to stabilise herself, which did include removing the risk of future pregnancies as that's a big trigger for her psychosis, but it wasn't a step she was willing to take.
Nobody can say exactly how this will go for you. If father is going to take custody but reluctantly, it's realistic to think he's going to need to do this as easily as possible, so it's probably going to be formula and lots of help from his family. It may well be wise to focus on stabilising yourself rather than the absolute horror of constantly expressing; but it is your call.
You're in my thoughts x