A photo book of school to keep at home is a fairly standard offer - but something that worked for us was to have a home photo book kept at school, in their drawer, so they could go and look at it whenever needed to help calm and soothe with familiar pictures (of favourite toys and of us together - with a reminder in that I would see them soon).
Another one that worked for us was back-chaining. Instead of going in for the start of the day and coming home early (when no-one else is coming out), we did a phased introduction working from the end of the day. To start with, they went in for 15 minutes of story time at the end of the day, then the last activity plus story time. The possible difficulty with that is whether there is a slight element of chaos to the end of the school day or whether it can be a calming, quiet time (maybe with a smaller group). Mine was often led out just a minute or so before the others so they 'found' me before the general melee.
Starting in September after a 6 week break is always tricky so maybe a visit the day before school starts (usually a staff training day) to hang up a PE bag on their peg or put something in their drawer ready for the next day?
I bought a miniature version (not an exact copy) of a special teddy and attached it inside their school bag so they had to bring it home to match up with 'big ted' each day. It depends if attachment and feeling secure are issues?
Something else that may help is an object, same one each day, that is hidden by the teacher each day after school for your child to have to find in the classroom each time they go in - can start on the transition visits. It can help to make that transition across the threshold for an anxious child if they know there's a hide and seek game waiting - and also might help them to know the teacher is thinking of them when they are not there. Takes less than a minute for the teacher but can help the transition into the classroom and also help with learning their way about a bit.
Will there be any 1:1 time provided? If so, having a definite, set in stone, timetable for the first 15 minutes and last 15 minutes of a day can be very useful - so they learn for example that their day will always start with a 1:1 story in a quiet spot, followed by a jigsaw puzzle with 1 other child and then a table top game with a small group (a 123 approach to avoid being overwhelmed by too many people when still struggling with the transition into/out of school time - equally important can be the 321 at the end of the day).
Also, try to get a dysregulation detective approach in place in school - talk to the SENCO during her visit about the subtle signs dysregulation is starting and how they build (often missed in a group environment) the agree a de-escalation strategy to be consistently used. So, spotted early at the time of flushed face and a bit of hand flapping (whatever first signs are) - verbal prompt from adult of 'You look as if you need a drink (or whatever works), let's go and find you one'. If first signs missed and they see snatching of toys, louder voice then response is ... and so on. They need to know what to look out for and what helps to calm and relax your child once in that zone.
If possible, a postcard or similar to arrive the last week of the school holiday saying they are really looking forward to seeing them and they have ... ready for them (something they know is enjoyed).
Hope something in there helps.