Definitely worth trying to memorise things like 'Gimme 5' or other such mnemonics: e.g. tone, single-word connotations, imagery, alternative interpretation.
If you're struggling with the general essay writing too it might be worth checking out structures like PEE, or the more complex ones. They might not get you to an 8 but will certainly get you up higher most of the time.
Another thing I'd recommend doing is checking out exemplar answers: there are loads on YouTube and the web. It's worth looking at these and seeing how they structure there arguments, what they're looking at, etc.: you can also find some really good ideas.
Having good quotes is vital. Depending on what books you're doing, there are some that can cover a wide range of themes/questions, but in memorising these and memorising your annotations you can really help your essays.
Also though, structuring your argument is really key which is why it helps to plan if you don't already. For example, you might consider 1 thing in paragraph 1, and another in paragraph 2: having different ideas to backup your argument is important. But you ought to link them together, for example 'Macbeth is also seen as bloodthirsty later in the play, where he...'
Other things that might you might already be doing but are really worth it if you're not include embedded quotations, putting in context (but make sure it links to the question) and just general use of connectives like furthermore, additionally, however, on the other hand... They really help to give you more structure.
Hope this helps! It's very admirable what you're aiming for, but remember not to push yourself too hard (it's not like, too, Literature is needed to go onto Sixth Form if you dislike it). Best of luck
P.S. again if you haven't already, check out Mr Salles Teaches English and Mr Bruff on YouTube, they have some really good videos: the former is good for exemplar answers and essay writing, the latter is good for strong analysis of books and poems