In my school last year a Y6 DC with a teacher assessed level of 2c or 2b (can't remember which) in his reading didn't take the reading SATs papers. DC who have no real chance of getting a level 3 can be excepted. They are still recorded on published results, so there is no advantage to the school. But it is pointless putting a DC through what could be for them, stressful SATs with no chance of achieving a level 3, IMO. This DC joined the school in Y5 and got a statement of SEN weeks before the end of Y6 that the primary school instigated, so at least he had more help in secondary.
With this DC, your DD (?) what are the school doing currently to support her? Does she get any 1:1 sessions? Has the EP assessed her? It's great that her reading has taken off, but such a shame that her writing is so far behind. If you present her with writing that you have done with deliberate mistakes in it, missing capitals full stops, etc, can she spot the mistakes and see how they make reading the story more difficult? If she can stop other's mistakes could she begin to edit her own work?
I suggest you post on SN children as well, you may get some more targeted suggestions.
Sounds like she might have been assessed by an educational psychologist, then. They would be advising the school on how best to support your DD. I think you should meet with the SENCo and ask how you could best help your DD at home.
Hopefully others on here will have one good advice.
There is now no writing SAT ( as in test) as writing is assessed by the teacher over the year. If she is working at level 3 or above in maths she will need to sit this, the same with reading and the new grammar test. But she can be scribed or transcribed for in all of these tests. ( I have a boy in my class with severe dyspraxia, his writing is completely illegible but he is very intelligent so able to access the tests in terms of comprehension, his answers were scribed for him )
She will not lose marks in a reading comprehension test for poor spelling, so long as it can be understood what she intended to write.
I teach yr 4 and have a lad who has dyslexia & dyspraxia. He is a very able writer in terms of structure, but it's totally unreadable. He therefore types on a 'Neo' which is a mini wordprocessor. It's great. His spelling is still unique, but at least it's legible! Would your DD's do better at typing than writing?
We have co-writers, which sound similar to the Neo. A child with dyslexia/dyspraxia wouldn't use them all the time but enough to ensure that we are assessing the content rather than the legibility, or not!
There's a new SPAG test (spelling, punctuation and grammar) which started this year with the current Y6, which your DD would struggle with. The reading comprehension paper wouldn't be marked down for spellings etc, but the school would have to ensure that your DD's work could be understood well enough to be marked. I have seen DC with dyslexia having the paper checked with them at the end and any difficult to understand spellings written out correctly above them to aid marking, in the teacher's hand, with it made obvious that it was done by the teacher for this reason.
But, TBH, although it would be good if your DD gets a SATs result that is appropriate for her ability, you wouldn't want any help to artificially improve her score as that wouldn't help her to access the support she will need in secondary. A big discrepancy between her reading and writing 'nicely' highlights her difficulties and should be good evidence that she needs support.
Sorry, forgot to update the thread before I posted and it's moved on a bit.
My DS2 used an Alphasmart Neo in primary school, Y3 - Y6 and for his SATs. His typing was much quicker than his handwriting. He has ASD and is quite dyspraxic. It's a simple word processor but wouldn't help with punctuation. There is a spell check function but he wasn't allowed to use it during SATs.