If you read through my old posts, I'm quite anti-mixing. I'm not totally against, but I think there's a lot more negatives than schools present to you.
All mine went though infants (2 form entry) and then Juniors (6 form entry)
Dd1 went through school never mixed up (except when going infants to juniors)
Dd2 has been mixed every year except going into year 1.
Ds has been mixed every year.
Dd2 was the first year to be mixed (at infants) because there were two forms that were very unequal. One had the high-fliers, and the strugglers and a lot of very confident to the point of bulshiness children, plus a few with behaviour issues. The other form had a lot of middling quiet children who would always get on quietly.
I think mixing them did help in a lot of ways. There were also a few children who clashed, so they could separate them.
From that, I think mixing them at the end of reception would work well, to give two more equal forms in academics and behaviour. And then perhaps going into juniors (if primary, obviously infant to juniors do anyway)
Often one positive is given that the children get a greater friendship group, which is helpful in going to secondary.
From observation some children get a bigger friendship group definitely. However this is very much those who were already confident in friendships, it also has meant that they tend to band together so they become a more dominant confident group who has no use for the others. Those who are less confident/find it hard to make friends are usually just beginning to really feel comfortable just before it's time to move round again. It also causes them a lot of worry.
Having observed various years from before mixing, to the years that mix, I would say there is no difference in the worry level about moving to secondary; the worriers, worry and the confident ones don't mind. Again, it's the confident ones that find that they know a person in their form fine, but are happy to make friends with others. The less confident ones still only have a couple that they really would like, so unless they're very lucky they still don't have a friend waiting at secondary.
Also from observation, most friends, even very strong friendships that are split up generally don't last. So there is the aspect I feel the teachers are playing "god" with the children's friendships. "They can stay friends/ they can't".
Usually one makes a best friend first (again, often the more confident of the pair), leaving the old best friend in a bit of a state, which sometimes they never really seem to recover friendship wise.
There also (in our juniors) is no way once they've been split of requesting they go back together. They can name 10 people from their own form. For most of the children, if they get 1 or 2 off their list they're happy; if they get 9 or 10 they're looking at fairly indifferent by that point. Again, those with a bigger friendship group to start off gain by that. (and if you think about it most children will tend to name mostly the same gender as they are, so that's roughly asking them to name 2/3 of those)
The other problem with if you're split from your friend is that because they do generally take friendships into account, you can find they have to break into friendships to make new friends. So you're not starting on an even keel.
Looking at the children I have seen go through. Some have stayed with the same friend right the way up, and have remained friends, those who have been split with the friend, I can count on the fingers of one hand that are still very good friends.
And (having had one) staying best friends with someone in another form has its own problems. Means that they don't have a natural partner for school trips, PE, academic projects etc. In my dd's case it actually restricted her friendships because she spent lunch/break with the best friend, so the others from her form got into their friendship groups and she was an oddment in her form.
For my dc, I was glad dd1 wasn't mixed up because she went through enough stress just about moving up every year until she went into year 6.
Dd2 it's been dreadful since juniors, 3 out of 4 years she's been split from any friends-or rather she's been given number 9/10 on the list. The only year she hasn't been is the year I went to point out that the previous mixing had put her with the child (previously from a different form) who had been bullying her. She now feels everyone hates her and has totally lost confidence in friendships because she's always split up. Until juniors she was very confident socially. Socially she is a shadow of what she used to be. Out of school she's fine, always the child who comes back from a day out with a new friend. In school she is now deliberately excluding herself and hiding away, and is suffering anxiety.
Ds couldn't care less as long as he can play football at lunchtime.