Namenick,
Yes. Also before secondary. I don't think you can cope with secondary without understanding place value and division.
I agree that no solution would be without trade-offs, but grade retention at some key stages would allow to keep the access to good qualifications open and maximise the overall utility, i.e. outcomes, efficiency etc.
Absence of some skills at the age of 4 becomes a bigger problem at the age of 6 and an even bigger problem at the age of 11 and so on. So it is better for these children to address that first.
Retention at key stages of development for a small proportion of children is a better trade-off than charging ahead in full knowledge these children won't make it to good GCSE and beyond.
I think retention in early years would reduce the proportion of strugglers later. So it will not be like 35%. It would also solve the problem of the more able, if they start at 4.
I also think for the 2-3% of genuinely gifted there could be other options available in secondary.
And concerning PE and PSE, some of those children in primary are less mature anyway.
Concerning Further Maths, the French model does not depress maths excellence. Equivalent is Bac Scientific, which 27.5% pass. Grade 19/20 is probably as rare as 98% raw mark grade 9.
The case of two additional years of maths after Bac is irrelevant - Not additional money, they would be spent anyway, as it is a very specific case of being first two years of the Master course in selective Granges Ecoles.