In terms of how many units you should do,12 units in year 12 may or may not be too much to handle. The main benefit of doing 12 units in year 12 is the availability of a backup subject in case something goes wrong with a particular subject at the HSC. On the other hand, Mathematics Extension 2 is a demanding subject, meaning you will likely have to allocate a considerable amount of your time to it, which may result in you not having enough time to spend on your other subjects if you were taking 12 units. In that case, doing 10 units in year 12 (that is, one less subject) may allow you to spend enough study time on each of your subjects. However, if you're very capable at maths, then doing 4U may not be a problem for you, allowing you to spend additional time on your other subjects, which in turn allows you to effectively handle 12 units in year 12. Alternatively, to avoid any uncertainty, you could go into year 12 with 12 units and see how you go. If 12 units end up being too much, you can always drop to 10 units.
I think that Physics and Engineering Studies are both beneficial subjects in terms of your career aspirations (which I remember you discussed in an earlier thread). Physics is a subject with a more scientific focus whereas engineering provides you with a foundation of knowledge in the field of engineering. In that case, you may wish to consider other aspects such as which subject you are likely to enjoy more/may be less demanding as well as which subject scales better (I believe physics scales better than engineering).
I hope this helps!