Reminds me a bit of my daughter. Good content, adequate handwriting, appalling spelling - her grandmother (with a background in remedial teaching) spent a lot of time with us when she was in year 3 and 4 and helped her learn her spellings for tests, but it still didn't show up in written work. Her teachers weren't bothered about it at all until year 7, at which point I was relieved to no longer be the only one worrying. I definitely recognise your sense that she doesn't have a visual of a word "looking right".
We did get some assessments done a couple of times but she didn't meet thresholds. I think she had a bit of a spiky profile, but between normal and very high in different subtests. Interestingly her father also recalls being pulled up on spelling issues in year 7. But her brother like me found good spelling instinctive.
She was on an academic scholarship from age 13 and is now nearing the end of an English degree. Though her text the other day, having just finished a major essay, acknowledged that the last spellcheck before submission was rather critical ...
So, what worked? I think knowing spelling rules is important (eg doubling consonant when adding ing after a short vowel but not a long one, that sort of thing); understanding groups of words with similar patterns - which you can also extend through looking at the origins of words (my daughter is very interested in etymology); but most of all accepting the reasons why correct spelling matters eg homonyms and so on. Acknowledging that sometimes mistakes happen and you might have to take extra care to spell things right. Drafting, correcting and copying out the correct version. Learning how to type and how to check your writing on a computer - how to spell check but also what words to watch out for because the spellcheck will miss them. Knowing that things may go wrong when you are tired.
Good luck from here, I hope you can get your daughter some effective support because I think you have to get the balance right between encouraging them to just write, to be able to get their ideas down on paper, and supporting them to write "right", getting a sentence down correctly first time.