I think a frank and honest conversation is needed about the reality of you going back up to FT hours. And what it will mean now that you are a larger family.
So your time currently is spent doing X hours at work, Y hours on housework and chores, Z hours doing child-related things. And you have to sleep so you realistically only have A hours for personal time. If you increase the work hours to X+8 or X+16, depending on 4 or 5 days, where are those extra 8 or 16 hours going to come from. And what compromises as a family will be needed to allow that.
What flexibility that you currently have will you lose, to deal with Drs or other appointments during the working week, school meetings (and all the annual shows, parent/teacher meetings, etc), dentist trips for you and DC, any business appointments needed like occasional trips to the bank or whatever?
What is the plan for afterschool care if you are working? That is an extra cost to factor in. And it becomes trickier as DC get older and many of the potential afterschool activities they could do are midweek afternoons when you will be working (sports of all kinds, drama, music, scouts, cookery school, book club, ….depends on your area generally and also on your PTA for what’s offered in school) - many afterschool clubs only want to collect DCs immediately at the end of the normal school day, won’t facilitate activities afterschool either at school or elsewhere.
What will happen all the household chores you do?
I would also have a similar XYZA accounting of hours for DH, even if approximate, to show the balance (or imbalance) currently and how his numbers would also have to change.
Or will you need to buy in services like cleaner, afterschool, food prep boxes, online groceries and delivery fees,….? How much Willy hat cost, and take away from your increased pay?
And if his plan is that you use the extra money for his hobbies and big holidays - are you getting even less leisure time? And will you have the holidays available from work to go away that much, especially as you’ll need spare days for school events and how will you cover school holidays as they are longer than annual leave allowances usually. Camps for younger DC are hard to find and expensive, and even for older DCs, they tend to be 10am-2pm rather than full day.
Ask lots of questions about what his solutions are for these problems - have some ideas, but try to allow him to come to the conclusion himself that it’s not feasible for you, at the present time, to go back FT. And also to come to the conclusion that his plans for rock climbing and IM training will negatively impact on your (probably already small) leisure time - so apart from the money considerations, he needs to think about the fairness of that. And that he expects to find these hobbies by you both becoming the higher earner - and losing out even more on the things that matter to you and to your ability to have time for a hobby or 2.
So what will he start to do instead (from your Y and Z hours) to help offset the increase in X and forced decrease in A?