My wife and I have been reviewing homeschool curriculum for my daughter's upcoming 4th grade year. Here in Utah the laws are very relaxed around homeschooling: you can pretty much do whatever you want as long as you tell the state you are homeschooling. I'm not sure I love that, but it does allow for flexibility.
What we've been finding is that nearly all the high-rated courses are non-secular and we've decided we want to keep our kids in a secular curriculum. We've evaluated trying to edit the courses or pick-and-choose but it seems like in many of the offerings religion is so deeply-rooted in the work that it would be a very difficult undertaking.
My wife feels like the prospect of designing her own courses is very daunting and so it made me wonder if there has been any kind of effort to open source homeschool. Now more than ever it seems like a great time to have something like this available (we don't want our kids in a classroom while COVID is a concern). So far I've found mentions of open source schooling but nothing concrete or usable.
What I don't know is: what challenges might there be to creating such a thing? In Utah it seems very usable, but I'm sure each state has their own homeschooling requirements that maybe make this idea unlikely to take hold.
I think you should put aside the quest for "open source" curriculum and choose the best items you can find a la carte. The point of homeschooling for me, as a secular homeschooler, was to create a customized educational experience tailored (meaning adapted on the fly) to what each child seemed to need and what seemed to work. That meant not looking for comprehensive curricula but trying different things for different subjects. Keep working with other homeschoolers, online and off, to find materials to try. It's very difficult to predict what will work before you try it, because kids are so different. Reviews and recommendations help, but steel yourself to the hard fact that doing the best job will mean expensive trial and error.