For goodness sake please take this as a red flag and whatever you do going forwards, look after number 1.
He thinks his ex had it easy for 5 years doing full time childcare 🚩(and presumably most of the household and mental load because women tend to do that especially as the SAHP).
I will look at working part time. 🚩
Please don't mess with your own earnings and future career. Women who become mothers often do get fucked over financially - be smarter than that.
He was always adamant he wanted to be hands on and would take extended leave. He’s now decided he doesn’t want to 🚩
Red flag city. You got pregnant and planned your life around this scenario, now it's too late to back out, he's handily changed his mind. It's a dick power move.
Did you discuss how he would be financially compensated (including pension contributions) as the lower earner taking extended leave?
I'd put money on him not having topped up his ex wife's pensions while she was at home.
If the higher earner has to take leave (and fuck with her earning potential) they can't afford to live without drastically reducing bills, and he sure as shit won't be topping up her pension, will he.
OP, I would look into childcare and decide if you want to go for nursery or a nanny (latter may suit you better, assuming it is affordable for you). You can present him with two options: He pays 50% for the childcare, or he can of course reconsider being a SAHD. What he can't do is fuck off his responsibility for the child he fathered.