but I do disagree that homework in primary has weak effect
I didn't say doing work at home had weak effect, I said the evidence for school provided homework is extremely weak. ie if you look at the studies of this they don't find a strong correlation between homework and success. Of course it's an extremely difficult thing to study, how do you control for people who do the homework, what else they do etc.
Because homework has a negative on time spent practicing other things, I'd want to see large benefits in the studies that just looked at academic outcomes - but that certainly isn't there - and there's almost definitely no dose effect more homework doesn't lead to better outcomes.
Tightly defined, targeted work at an area of weakness for an individual child, probably useful. Minimally differentiated work continuing what was done in the class, I can't imagine the utility.
I'm far from against working at home, but then I fully believe intrinsic motivation will direct most children into learning what is interesting and useful to them, and providing time for that is valuable, if you're a kid who needs 11+ hours a night sleep then there's not a lot of time if you eat up some of those hours with homework.
Of course it's easier to not support school homework when your child finds it all easy and is intrinsically motivated to learn themselves as DD is, and obviously with personal experience of rarely doing homework even in secondary and still over-excelling at school biases my own views. That's why I looked hard at the evidence with DD (DP also excelled at school, but did much more homework, but different countries and parents)
My conclusion was targetted individual homework that takes a short time and doesn't detract from something else (ie not if you're missing out on an hours physical fitness to sit at a desk working) is probably good.
Learning spellings that the whole class has is not targetted, therefore I cannot see the value, obviously the child who can spell them without effort is not getting any benefit, and the child who spends three hours and still fails has wasted a lot of time.
If the teachers cannot provide those individual targets (and I wouldn't want them to unless as an intervention) then having the child lead, or just doing something fun would be more productive.