I teach a 1/2 mix, but in no way consider myself to be a year 1 expert. Also, because of the year 2s in the room our day is much more formal than it should be at this stage of the year. I do my best to make the activities age appropriate, but there are considerable resource constraints which make it impossible to run year 1 as I would like. So I won't answer your questions about routines!
Changing reading books - they are all in colour coded baskets just outside the classroom. Since the start of term my TA has been helping the year 1 children to change their books each day but that support is being gradually withdrawn so in a week or so they will be reminded to change their books and be expected to do it themselves. I work on the basis that I don't want to read a book someone else has chosen for me, so why should a 5 year old? Obviously, we are already working out which children won't hear the instruction to change their books and will then need a personalised prompt! Also, I've explained the system to the parents and encouraged them to check their child's bag when they come out at the end of the day and if necessary send the child back in/take them back in to change the book. That's where a mixed age class is a bonus - they see the year 2s changing their own books and they can copy them.
Writing - I've got everything from those who can only just write their own names through to a couple who produced half a side of A4! I'm doing lots and lots and lots of modelling - what is a sentence? Then getting it wrong (ie capital in the middle of a word, forgotten full stop etc) as I put it on the board so they can spot the mistake. They love that! Once a week we are going to read a story then they will have pictures to sequence, then write a sentence about each picture. I'm also using the mantra "think, say, write, read". Meaning think a sentence, then say it out loud to yourself, then write it down, then read it back - does it say what you want it to? Repeat. Repeat. Repeat.
Maths - everything from a child who doesn't securely recognise numbers to 10 through to those who can add a 2 digit number and a 1 digit number. We're keeping it very, very practical. Lots and lots and lots of visual, practical, concrete apparatus. So last week they added various items in split pots (like you might get 2 things from a take away in), dots on dominoes, rolled dice, spun the giant spinner and anything else I can think of. They were encouraged to put the big number in their heads and then count on - most of them would count the things from 1 side of the pot onto the table, then count the things from the other side of the pot onto the table so they had a bit pile, then starting counting them all over again from 1. We're trying to get the more able to realise that if there were 5 in one side and 3 in the other, they can start on 5 and count on 3. We're also starting to introduce the more able to the written notation of a number sentence. Lots of using number lines in various forms for them to count on or back on. I've got a giant floor numberline jigsaw so they can make it (can they order numbers?), start on a number, generate a second number (dice, spinner etc) and then physically jump forwards or backwards the required number, and see what they end up on. In the first week they made their own number tracks - they had 0-20 in an envelope which they had to put into order and stick on a strip of paper. These are now in the front of their Maths books so those that chose to can use it as a number line.
Does that sort of thing help? I feel your pain! My TA and I look at each other boss-eyed at 3.20 every day and collapse. This time of year is hard, hard work. But it does get better, honest! And it's a fun year group to teach as they are so enthusiastic and make so much progress.
If the lack of structure is getting you down, perhaps you could work towards a more formal morning (English and Maths) and a more play based afternoon? I know there are some very experienced KS1 teachers on here though who will have much better advice than me about how to make child initiated learning work in KS1.