I think there's actually no way of making it more fair
There is, but it's pretty much impossible outside a world of giant super-sized primary schools, and completely new staff contracts, and fighting an all-fronts battle with parents who want to go on holiday.
You start all children in the month of their fifth (or whatever) birthday. September children work roughly the current pattern. November children work very slightly longer terms, totalling an extra three days per year. December children work an extra six days a year. All the way through to August children, who work an extra 30 days per year, getting only six weeks instead of twelve weeks holiday (you see the problem).
They all arrive at secondary school having had the same amount of teaching, at about the same ages all the way through.
You could play tunes on this and work in terms of teacher-hours (ie, a child working 30 hours a week in a 30 pupil class counts 1 teacher-hour) so that the number of teacher hours is the same by year 7, which would involve the August stream being a bit longer in terms of hours and also a bit smaller in terms of class sizes (ie, needs more staff).
it's basically impossible, outside very large cities, because it requires 12 streams, so either a super-school which doesn't exist or a federation of four to six standard sized primaries.