I read some of the thread & all OP's posts so apologies if all this has been said!
You are missing the point wildly OP.
If you previously worked in the office, as it appears, then WFH has been an anomaly, albeit one that surprisingly extended for a long time. (Most workplaces moved to hybrid at least 2 years ago).
You are still benefitting from 3 days wfh, and flexibly at that as it seems it enables some collection of your DC, and presumably organising a snack & supervising them. Great if that works for you & employer.
Going in 2 days a week is not about career progression or at least not mainly. It's about commenting as a workplace community, allowing natural & on the spot conversations which are less easy online / phone & in general creating a cohesive workplace. Hybrid / blended can work - the on-site elements are almost always important for efficiency.
I'm sorry your ex is paying insufficient maintenance. I'm a single parent, I get it. But it's so unreasonable of you to assume that's a problem for your employer. Childcare is a condition of work.
If I were your manager I would not agree to you making up hours from home. This is primarily for your well-being. You need proper down time, with your DC & resting / personal time / sleep. It's also because any workplace needs at least some agreed boundaries - what if someone decides working 12 am - 7 am is actually what suits you?
I think asking to work fewer hours is reasonable & I'd accommodate that. But you say that causes issues of affordability.
It's time to face up to ex not paying sufficient maintenance & take it from there.