I'm currently doing extension history and honestly it is a lot of work because the style of the course is EXTREMELY different to ancient/modern history. The first lesson we had we were given a stack of work from a historian and about 20 questions which covered all of the content to give us a taste of what the course was going to be like. The course work is also intellectually challenging because what you do isn't just black and white on the page, its about thinking outside the box abstractly to convey and prove your understanding, interpretations and opinions on generally the purposes of historical works.
The course is "harder" than other HSIE subjects and will have double the work as what you do in ancient in half the time but once you get a grasp on the way the course works, which can be difficult at first, A LOT of it is just applying the same process to different sources. It's only content heavy because you need to be able to read and understand a lot of historical writing and make deductions about why it was written, if there were any bias / external influences on it and being able to identify historical and ethical issues in work, all which gets developed through all the case studies you do through the year (take a look at the syllabus and supporting docs on BOSTES)
The project also doesn't have to be extreme, you need to know what exactly you are interested in and work out how the syllabus applies to that event / time / etc. Most schools give you the project to do over the holidays and you need to be able to put aside a few days to just sit in a library and read your main focus works & research online, from there there isn't much left on the project (essay is only 2500 words iirc? with a few other parts).
I will say that many people apply what they learn in SOC with the ext course which to an extent is useful but you will need to be able to ignore that for the most part and relearn how they want you to view things in this course.
It's also good if your school does it as a period 0 class or off the timetable because when its a hot day and you've already had 3 other classes there's no way you can process the concepts in class, my school does it two mornings a week and that's honestly a great time because you are fresh and you are able to take it all in. However you don't need copious notes and details on everything because there is only a limited amount of questions they can ask you, the most part of the work will come with your core historian and once you're deep in the course its a lot easier to understand what you need to know and how to apply it to the question (sort of like manipulating the questions in ancient if you've learnt that)
I'd say pick it up, its a one unit class and if you can perform well in it it will be good for your ATAR, the projects worth 80% of your internal (40 really because its a one unit) so if you're able to work hard on that you'd be set and if you feel like you're struggling or not enjoying the course then you can drop it and focus on your other subjects.