I'm a single parent who has always worked and work more than full time hours now, and has a child with very complex medical needs and SEN. I also have other dc. This means a lot of appointments and has shifted how I work, where I work and how I use my annual leave, but just because someone is a Mum it doesn't mean they have to work less.
If I had the choice I would work 4 days/week long term. There's nothing wrong with making that choice. I too sometimes eye roll a bit at the 'I have one child who is only 8 and can only work 12 hours a week because being a mother is a full time job' brigade. Women can and do work far longer as mothers. Having said that, if you make a choice to work less than full time to priortise other things, that's a very valid choice. That extra day can be a chance to have more work/life balance, rest, catch up on chores, when all this is over you could volunteer at dc's school, have appointments out the way without interrupting your working days. So many options.
While I do think women and mothers don't have to work less just by virtue of being mothers, and we need to chance that narrative away from women and more to how parents will address dc's needs, but any person and parent can choose to if it works better for them and their situation. If you can afford it, take it. It doesn't have to be permanent. Maybe you could go to 5 days when your dc is older if you want to. Sounds like he's only just at the start of school.
As for finances, couples approach this in a variety of ways. But I do wonder if paying an agreed % each month to contribute makes sense as opposed to an equal amount. That way it is more fair and doesn't leave you without access to savings. For example both contributing 75% of your income to all household and dc related finances. Allowing you to each keep 25% for personal spending and savings etc.
Also poverty is known to go either way long term, either not being able to make good money choices or making for more anxiety about money. This is well documented.
Good luck @RootBeerFloat43 hope you can do what you want.