@v.tex, there are eight points that I would like to discuss. Hopefully considering them alongside everyone else's advice will help you make a final decision:
1. Established foundation versus unchartered territory:
In your case, both Earth and Environmental Science and Chemistry have advantages. EES is a subject that you are familiar with and confident with, and Chemistry is a subject that offers higher scaling. Depending on student circumstances, some advantages may be more important than others. In your particular case, it seems that the advantage of superior performance in EES would outweigh the superior scaling of Chemistry (there will be a bit more information about this below), primarily because you have established a solid foundation of knowledge and have a good understanding of your current level of performance in EES (including your strengths and areas of improvement). As such, you likely have a clearer vision in terms of starting the EES year 12 course which, if everything goes well, should translate to virtually guaranteed favourable performance in the subject. On the other hand, while you could definitely perform well in Chemistry, you have no previous results that you can refer to when making a decision, which makes Chemistry a slightly more uncertain option.
2. The nature of the ATAR:
It is important to keep in mind that the ATAR is determined through your performance in your best 10 units. Based on this, you must ensure that you do not allocate excessive study time towards any one subject, as this can lead to insufficient study for your other subjects, which can consequently put your performance in your other subjects at risk. In your case, the subject that you end up choosing must be one that does not present a negative impact on your other subjects. From previous posts, EES seems to allow for a reasonable balance, while this is an aspect that is harder to determine regarding Chemistry because you did not take the subject.
With that being said, since you are still in year 11, and none of your results counts towards your HSC results/ATAR at this stage, is it not still possible to use the time that you have before year 12 starts to finish catching up on the Chemistry year 11 course (and possibly gain a general idea of the year 12 course)?
3. Achieving a high ATAR:
It is important to stress that a high ATAR (including 95 and above) can be achieved with any subject combination, including EES and Chemistry. The 2023 UAC Scaling Report was mentioned in earlier posts. If we have a look at it, we can find that the highest ATAR achieved by a student where EES and Chemistry were counted as part of the best 10 units was significantly higher than 95. Specifically, the highest ATAR achieved by a student who took EES was 99.80, and the highest ATAR achieved by a student who took Chemistry was 99.95.
4. Individual subject contribution to your ATAR:
Whether you keep EES or take Chemistry, favourable performance in a subject will always mean that the subject will make a positive contribution to your ATAR. You simply need to be aware that a higher mark would be needed in a lower-scaling subject to match the ATAR contribution of a lower mark in a higher-scaling subject. An example of this would be Mathematics Standard and Mathematics Advanced, where an HSC mark of 80 in the former would typically result in a lower ATAR contribution compared to an HSC mark of 80 in the latter.
5. Your study interests beyond the HSC:
In your initial post, you mentioned that a reason that you want to take Chemistry is that it is relevant to your studies beyond the HSC. Here, you should consider the ATAR/selection rank requirements of the degrees that you are interested in. If your 95+ ATAR goal is simply a challenge, that is, if the degrees that you are interested in applying for all require an ATAR significantly lower than 95, perhaps you could take Chemistry and do your utmost to perform well throughout year 12.
Alternatively, if achieving your ATAR goal is very important to you, you could probably keep EES and do your best to perform well across your subjects, which would maximise your chances of reaching your desired ATAR. Chemistry is not treated as a prerequisite for relevant degrees but is treated as assumed knowledge. To address this, you could work on catching up on the Chemistry year 12 course after the HSC, which should be possible with commitment, and thanks to the wide range of study resources available for the subject.
6. Past cohort performance:
The performance of previous cohorts does not determine your own performance. Instead, it provides an indication of the potential academic capability of your cohort across your subjects and, along with the number of band 6's/E4's achieved, can help you determine the internal ranks that will likely be needed for your internal performance to be consistent with that band range. Past cohort performance should not deter you from taking a subject (this applies if you decide to take Chemistry).
7. Achieving a higher HSC mark in EES:
Essentially, an HSC mark that makes a greater ATAR contribution will be better than an HSC mark that makes a lower ATAR contribution, even if the latter was achieved in a subject that scales higher (in this case, Chemistry). We can use your example of achieving an HSC mark of 90 in EES compared to 80 in Chemistry. Suppose that you achieve an HSC mark of 90 in Physics, Economics and English Advanced, as well as an HSC mark of 45 in Mathematics Extension 1. UAC Compass estimates an ATAR of 95.95 with 80 in Chemistry, compared to 96.50 in EES.
8. Importance of proper decision-making:
Lastly, a good decision is one that is rational and informed. You have set yourself a certain ATAR goal (whether it be to meet the requirement of a particular university degree, or as a personal challenge). In addition to committing to your studies throughout year 12, maximising your chances of reaching your target ATAR will depend on the decisions that you make. As a general example, a student who keeps and continues to allocate study time towards a consistently lowest-performing subject simply because it is a backup subject (or another unimportant reason), instead of using that time to study for their other subjects (and perhaps dropping the low-performing subject), can prevent themselves from achieving their ATAR goal, making this an irrational decision.
I hope this helps!