Doing a B. Economics in addition to B. Commerce does open some doors, for example, government economic institutions such as the RBA would be a lot more difficult to get into if you didn't do an economics major. (Mind you, a lot of B. Commerce degrees allow you to do a second major in economics anyway. So if you don't have a second major in B. Commerce, just do an economics major under B. Commerce.)
However, in general, economics is more of a theoretical discipline. For most people who study economics seriously, the beauty of economics is first and foremost in the abstract theory, then real-world applications. I think it's important to stress that fundamentally, economics is an arts and social science area of study (for example, I'm doing a combined Arts degree, and my Arts major is economics [USYD really doesn't try to sugarcoat that economics is an Arts subject lol]), and not a business or commerce subject. The kind of economics I'm currently gearing towards (for now) is really just an extension of applied mathematics. Even if you decide to enrol into a unit like environmental economics, it's going to be partially essay-based, and partially maths based. Behavioural economics, for example, sounds like it's going to be psychology, but actually, it's mostly just applied maths (e.g. how people make choices under risk, across time periods [for example, hyperbolic discounting model]).
If you're worried about the maths, don't be. Doing Extension 2 Maths is enough for at least first-year and partially second-year economics, and things like partial derivatives and optimisation will be taught to you at some point in uni, if your courses need it.
From my experience, most of my workload in economics has been the content/topics/theory. But in terms of things like tutorials and assessments, economics has the least workload for me (most of my economics units tend to have, say, a midsem exam and an essay during the semester, so literally two assessments, and the tutorials are short and manageable). This is in comparison to the other units I do, like maths, stats and computer science (my maths degree). I think a motivated student such as yourself should find no problem in coping with the workload of economics units. (That said, understanding the content is very important. But that shouldn't be difficult either, as long as you can read maths.) (Note that first-year economics have considerably less maths than second-year on-wards, mostly because HSC economics actually does not resemble economics as a discipline at all, so universities tend to ease students into it. But your exams for both microeconomics and macroeconomics should still be 90%+ calculation based in first year.)
I haven't had any experience with internships yet (I'm a second-year), but generally you don't really need extra-curricular activities for them. The main thing is to keep your marks high enough (at least a distinction). Most unis have internship programs, so I suppose they're first you should approach when the time comes.
Hope this helps!