Is there another subject that you are taking that you did not mention? From the information that you have provided, it seems that you are taking 11 units instead of the minimum 12.
Regardless of this, Biology appears to be an important subject to take if you wish to study a degree such as a Doctor of Dental Medicine or a Bachelor of Oral Health. The same can also be said about Chemistry. Both subjects are treated as assumed knowledge for such degrees, meaning that, while they are not prerequisites (i.e. you can still receive an offer to those degrees even if you do not take these subjects), it is expected that you will already be familiar with basic concepts covered as part of each degree. The knowledge that you would gain prior to studying either degree would therefore come from completing Biology and Chemistry, ultimately allowing you to start your degree comfortably.
Based on this, if you wish to drop PDHPE because your experience has so far not been positive, you can do so without any problems. In terms of the subject that you should take in place of PDHPE, you are free to choose any subject, since you are already taking essential subjects for the degree/career path that you intend to take, that is, Mathematics Advanced, which satisfies the prerequisite in the case of USyd, as well as Biology and Chemistry, both of which are treated as assumed knowledge.
Finally, when choosing a new subject, it is important to consider a subject that you are interested in taking and in which you strongly believe that you can perform favourably.
I hope this helps!
