Largely the evidence points towards it being harmful to learning in early educaiton.
> On the other hand, it is mostly intended to promote a certain type of work ethic where those who devote additional time and effort have more opportunities for success.
I think if you ask what homework is "intended" to do, you'll get a million different answers from different stakeholders. I think whether you assign homework or not, there's a massive impact of work ethic upon educational success and attainment of mastery. Some students just go the extra mile and are more successful as a result.
> Grades really should be abolished for more descriptive assessments,
I like grades for MS and up. As much as you talk about the confounds of grading, every criticism you've levied would apply even more to more subjective, descriptive assessments. But I do support emphasizing them less and trying to make what they measure truly be mastery of material.
Do you have a sense of whether that's harmful at the margin (a small reduction from typical produces better results), harmful in total (our results are presently worse than if there was no homework at those ages), or harmful in any quantity?
> harmful in total (our results are presently worse than if there was no homework at those ages)
Most studies compare schools / classes assigning no homework to various "normal" levels of homework, so they imply that our results are presently worse than if there was no homework.
I do assign homework to middle school students. But I don't do it often, and it's typically of the form "think about an idea on the topic of _____ we can discuss" or "do the first couple items of this lab worksheet so that we can use our time in the classroom more efficiently tomorrow." I enforce its completion with social pressure, not the gradebook. "Dude, you were supposed to do the first couple questions on the lab worksheet!"
I have a feeling that there is better homework we could assign that would be useful. But we don't have a lot of evidence of what that would be. Proponents of flipped classrooms think that is the path forward, but the research quality is very dubious in primary education.