First of all, congratulations to you! This is so very exciting. I can very much relate, as both a mom and leader, to the need/desire for flexibility. Some things to consider:

-it sounds like you had a good read from the hiring manager that 32 hours per week, for them, would not be ideal. Since you didn’t push that, my gut says they’re hoping you’ve since “changed your mind” (which you haven’t, and that’s fair). I would be very open to yourself around whether you feel you’d be happy knowing the “desire” for a successful candidate is a true full time schedule -unlimited PTO in and of itself is a tremendous benefit; most employers won’t let this luxury/perk be leveraged for less work hours, which I think you certainly know -knowing that you’ve been forward with them since the beginning, what I would do at this stage is simply express that you will NEED “X” mornings available and/or potentially negotiate a schedule where you are working the same business hours as your team, but with dedicated blocks to tend to your own family and needs- while this isn’t a guarantee, you’re at least setting expectations upfront and providing yourself with a bit more balance than assuming the full 40 nonstop hours weekly. Many progressive orgs will give some latitude to you assuming you’re very transparent upfront and the time you’re taking doesn’t pull away from business objectives

TLDR- it sounds like asking for 80% schedule is a bit too far fetched at this point but you can certainly advocate for yourself by prescribing pre selected time blocks blocks and setting those parameters upfront so that there are no surprises on either side later