You see I don't think the 12 months age spread is the issue. The issue IMO is that teaching (expectations/format) is aimed at the older half of this age range. If it were aimed at / appropriate for the younger half, then these kids would still experience being less able than their older classmates, yes; but they would NOT experience that school goes over their head, school is not for them, they 'fail' at school. They would not miss out on learning (despite being present) from being too young/immature to access that learning.
At age 4/5/6 half a year makes a big difference, less so the older the children get, so if you started formal schooling later, then it would be easier to 'pitch' the teaching.
But starting at age 4/5 IMO it is harmful (for some) to pitch the teaching at the older half. Pitching it at the younger half would not be without it's problems either - some kids may feel under-challenged. But IMO just because a child is capable of 'formal learning' does not mean that 'formal learning' is the best thing for that child at that moment!
It seems to me that if you insist on starting young with a 12 months cohort, then you will always be doing 'wrong' for some children - some will be under-challenged, or some will be out of their depths. But being under-challenged for a few early years (until things even out - the age gap gets relatively smaller) IMO has less long term negative effects than feeling out of your depth for the same few early years does.
And the under-challenged problem could be addressed by offering early starts/acceleration, whereas deceleration/entry deferral comes with a host of problems (people choosing to do it for advantage, poorer people not being able to afford to do it despite it being best for their child).
So if you are going to make very young children start school, then make school appropriate for their age. Make reception appropriate for 4 year olds, and Y1 appropriate for 5 year olds. Those who are nearly 5 upon starting reception/nearly 6 upon entering Y1 may feel that they would be capable to deal with more formal learning. But just because they would be ABLE to do more formal learning, doesn't mean that it would be best for them. And if their parents DO worry that they are missing out/being held back by being in a play-based environment in Y1 at 6, they can choose to send them to school early/to accelerate them.