Make a spreadsheet, put finances on one sheet, household chores on another and childcare on the third.
On the financials sheet put all your incoming and outgoings as they currently are and then what money you’re left with at the end of the month to spend as you wish.
Then add a column of the additional incomings and outgoings you would have if you were working FT. Bigger wages, more money to put into pension, plus more childcare costs (especially for school holidays) and see the cost-benefits there.
Then list all the household chores you currently do and what your DH does on the second sheet. Add to that the revised list of what you would both have to do if you were both working FT (I.e. split the list in half and increase his household chores list so he would be doing equal to you).
Then on the childcare sheet work out how you would cover school holidays, sickness, doctors appointments, etc using your annual leave. You might want to assign a fortnight for a family holiday and some joint days off for Xmas/birthdays etc and then split each of your annual leave so you each have a week or more (depending on how many days annual leave you get) to watch the kids by yourself. Then what childcare needs you’d have to cover the rest.
Then, any time your DH brings up working FT say “great, I’ve been looking at that and want to discuss how it’s going to work, let’s sit down and have a look at the spreadsheet I’ve built.”
If he says no, email him the spreadsheet and say, “once you’ve looked at the spreadsheet and confirmed which weeks you’ll be covering childcare, which household chores you’ll be responsible for etc we can potentially move forward with this.”
He needs to trial doing more housework, cooking, cleaning and washing on top of his FT working week to see how he copes with spending what he currently has as free time (as you manage the bulk of housework) cleaning the bathroom etc. Does he do it? If not then you can say “how can I go back to work FT if you can’t even clean the bathroom after work? Your expectation seems to be that I will work FT outside the home AND in the home. If I go back to work FT then EVERY task inside the home will be split 50/50, so step up.”
The next day one of your DC are ill and need to be picked up on one of your work days, make him do it. He needs to realise you won’t be the default parent when you’re both working FT and that will affect his working day and potentially his career prospects (depending what line of work he’s in).
Whilst I don’t believe that a parent has autonomy to decide they’re just going to work PT if the other parent doesn’t agree, the FT parent needs to be fully committed to doing their fair share of childcare, housework and other tasks that the default parent usually covers if you’re otherwise going to be working FT.