DS1 (11) has dyslexia, so has taken a long time to catch up to average/ expected standards within the school curriculum. But the school curriculum can be very prescriptive and one-size-fits-all.
At 6 his writing was hard to read with poor, inconsistent letter formations/ spacing, flipping letters and very phonetic spelling. He got a Star of the Week late in y1 because he'd written a paragraph and the teacher could decipher it all for the first time. He's always found the grammar content of the curriculum difficult.
In home education, you don't have to compare to the curriculum which caters poorly for children being "ready" at different times. The learning experiences of reading, being read to, drawing, mark making, writing and fine motor play (e.g. picking up beads) are very important. In another year or so, the differences of maturity in readiness for formal learning balance out a bit and many make leaps forward as things click together. It's for this reason that dyslexia testing starts from 7, because it filters out the slow starters that just needed a bit more maturing time. You also have the advantage of playing to their interests.
Independently writing is a great thing.
Many children can write more than a sentence. Some still struggle.
There are lots of features to a sentence (hand writing, content, understanding) This sentence is a simple one, but its a foundation, and the satisfaction of realising you can make your point in writing is a powerful motivator to keep learning.
I'm not assessing where she's at here, just that the range at this age is broad and progress happens at different rates even within one class sharing a teacher.