I would love to still be PT and spend more time with my DD however FT gave us extra income, financial security for my future and dislodged the financial strength my partner held.
I went FT when my DD was 6, however not only is your younger 18 months, you also have 3 children. Even if financially it really helped our home, going back to FT nearly destroyed our relationship. We are just coming out the other side just over 3 years later.
I suggest you trial it this year without actually increasing your hours. That looks like him submitting a flexible working request, doing 50% of the school runs (mix to suit your FT hours), extracurriculars, cooking, meal planning, shopping, cleaning, washing, life admin, homework support, buying things the kids need as they need, school holiday cover (both time off flexed and booking holiday clubs). Be out when he’s done the school run after school, returning after FT job hours, not be available in holidays etc as you’ll be working.
Write down every single thing you do. Then divide equally. If you do between now and the other side of the 6 weeks holidays and everything generally works with him pulling 50%, then he has an argument. However if your household is like mine, he won’t see everything that gets done and the shift to both working FT will be far harder than a bit of extra money.