It is definitely possible to finish the Spesh course over the summer holidays, although I don't think it is necessary for you to complete every question in the Essential textbook in order to achieve that. Maths is all about conceptual understanding. Hence, my recommendation would be to read every chapter in the textbook thoroughly and think about the concepts until you 'get' them. If you are having difficulties getting your head around a particular concept, ask someone or search online. Of course, you may find it useful to attempt a question here and there to consolidate your understanding, but I don't think it is necessary at this stage to go through every exercise, and here's why:
1. It takes way too much time. The summer break only lasts for 2 months. The time that you spend painstakingly going through every question in the textbook can be better spent studying for your other subjects.
2. Your teacher will set homework from the textbook during the year anyway. You don't want to proceed through the year thinking that you've already done the work, only to find at the end of the year that you've forgotten how to do most of the questions.
3. The questions in the textbook do not necessarily reflect the kinds of questions you will get at the end of the year. The Essential textbook does have some pretty good questions, but they are often not the same in nature and style to those which VCAA sets.
If you are looking to do well in Spesh next year, I think that your aim should be to, as quickly as possible, get to a stage where you can answer every single question on an actual VCAA paper without trouble. As I said, it is definitely not necessary to attempt every question in the textbook in order to get to that stage. Once you get that stage, my recommendation would be to start spamming practice exams immediately, as I feel that, especially for a subject like maths, more often than not, the study score that you get is proportional to the number of practice exams you complete, rather than the number of questions in the textbook you attempt.