Hand writing can also be helped by things other than writing. Often low muscle tone can be an issue. Things like swinging on monkey bars can help to develop upper body strength. Also, a firm ball that can be squeezed can help to develop muscles in the hands and fingers. Kids who find writing tiring may need work on their hand and arm muscles.
Then, to help with the writing tidiness, it's fine motor skills you need. Lots of craft activities can help to develop fine motor skills.
I'd use Chat GPT or Google for a list of fun ways to develop fine motor skills in children.
Having strong abdominal helps too. Children, when tired, will slouch, lean on their arms and again, feel fatigue when writing. Having stronger abs/torso can help with sitting up, not leaning on her arms and developing writing stamina.
This way, you can get her to work on her writing skills, without writing at all. In fact, don't tell her why you're doing these things at all, let it just be fun. Get creative, the ball squeezing can be done in the ad breaks while watching TV.
If you work on these things consistently, it will help with the writing mechanics. She may of course, still need help with correct letter formation when writing, let alone the skills to order her thoughts on paper, but don't dismiss or overlook the strength needed for the writing process.