Honestly, just chill. Dd could concentrate for hours as a toddler on things that interested her, but age 8/9 was still capable of switching off if it didn't.
Not a stealth boast but relevant, dd is academically able, and with hindsight always was. But while she started school years ahead in maths, literacy didn't interest her till she was nearly 5, and nationally probably started school at average, but she's a December birthday so older than your dd is when she reached that average stage. She had no problems with the motor control to write, but did v little, and tbh it came from writing numbers not letters. So I just left it alone and we did stuff she liked. Once she decided she was interested the concentration came.
Plus up until y5 or so, when it came to writing she was very hit and miss. If the subject interested her you'd get pages of beautiful and correct writing with a lot of thought behind it. If the subject didn't, a few scrawled thoughtless lines was likely. It was only close to being 10 when she realised there are occasions when interested or not you do your best.
Even now in y7, it's maturity that makes her do her best writing whether or not it interests her, not academic ability or a massive interest in writing about anything.
So by all means if you're worried about adhd for other reasons, look into it. But I wouldn't worry yet about her concentration or ability in literacy.