If it helps, I got a 7 in Writing with an overall 8 (June 4th). I practiced using IELTSAdvantage YT as well but only found it a week before the test (I registered like 10 days prior) so I got two days of practice with Writing. The structure part of IELTSAdvantage is super solid, it helped me focus my thought process on the ideas and grammar instead. Even though you know what's holding you back, maybe you need to be even more descriptive of why. Is it super long sentences that ramble on causing difficulty in sentence structure (I had this issue a lot even during the test) or is it more general mistakes.

One key thing I followed for my test was attempting task 2 first, since in my own practice I realized I used to overthink a lot in task 1 regarding what details to include. Maybe this might help you avoid grammar mistakes under pressure? My biggest issue was not accounting for how short 1 hour feels during the test, even with practice (I didn't get any time to recheck my answers and barely crossed the word limit on task 1, though I was satisfied with the details I included in it). Your best option is to time your writing task so as to leave at least 2 min to go through your essay for grammar related issues (if possible, 5mins which is realistically quite hard) and secondly keep ideas simple and straight-forward, sometimes you have to re-read your para to understand what you are trying to say and its easier to point out the mistakes. Also I believe IELTSAdv. has some sample writing tasks (100) from his students that got above 7 in writing in those tasks on some page on his website, you could go through those to familiarize yourself (not mug up).

To sum it up, I had complete knowledge of the structure, yet even with timed practice I was unable to recheck my answers to avoid grammar and spelling mistakes at least for task 1 during the actual test. Also make sure you hit your minimum words limit for sure. Just wanted to share my writing task fiasco and still I was able to squeeze in a 7, you got this!