I'm about to begin year 12, and one thing I've noticed is all year I've slacked off in maths advanced. I failed 2 exams in year 11, the others barley a pass, and I struggle understanding mathematical concepts - this is partially because of my laziness and lack of trying, but also I've found that I try to reteach myself content from the beginning of the year 11 and I still struggle to grasp it!

These are reasons to consider dropping. Past motivation is often a useful indicator of future motivation, and it doesn't sound like you actual enjoy or are interested in Maths.

One of my main concerns is whether I should drop my maths level or not. I may need it for uni, and that's why I am still staying in advanced. I've been in advanced course for maths pretty much all of high school - from 5.3 maths in years 9 and 10 and then advanced in year 11. I'm worried that if I switch to standard, there may also be gaps in my knowledge for HSC.

Standard has content that is not in Advanced which you will need to learn, but it does not involve calculus and is mostly more like year 10 difficulty. So, yes, there will be (addressable) gaps. Almost anything at uni wanting maths is going to want calculus, so you may find a need for a bridging course or a challenge at uni where the content will be assumed or be covered more quickly. This depends a lot on what you want to a study. A degree in physics or engineering will need a solid maths basis... a degree in psychology where you will need to study statistics can be approached more easily from Standard, while an Arts degree in ancient Greek and philosophy can easily rest on little basis in maths.

Is it worth it to try again relearn all of the year 11 maths advanced content for year 12? Or should I just take my chances and drop as I won't be able to do it in time with solid understanding?

You have a knowledge deficit to address either way. The more important question, in my view, is how you plan to address it. You've tried relearning on your own, without great success, so what would you change in your approach? The whole COVID situation has undermined motivation badly for many, but is that the problem for you in Maths, and thus it will reduce in time, or is it something else?

I've also paid for very expensive tutoring that goes a term ahead of the class content. The content seems very difficult, and I don't want to waste the money and resources the course offered.

I question the value of tutoring taking you ahead when you are behind in understanding, struggling, and lacking motivation. Maybe tutoring aimed at helping you catch up is worth considering? I also wonder whether this is placing the pressure of expectations on you, from yourself or others.

Bottom line, this question is a balance of what you want and what is reasonable / achievable... ultimately only you can decide, I just encourage you to consider all the factors you see as relevant before deciding about what is in your best interests.