Howdy,
I'm really not sure this is a good idea. I was somewhat in the same shoes as you - I did a long double degree, and in fifth year, I still had first-year management left to do. It was a pretty shit experience. A lot of the first-year unit stuff gets pretty annoying later on, and you're stuck with a bunch of first-years (do we still call them jaffys?) who don't really know how to uni.
Second, I think it's actually a good idea to try out those various first-year subjects early on in your degree, as they give you a taste of what's available in the degree. I would say this is the case even if you're absolutely sure you want to major in accounting, as you could discover a second interest or some interesting electives you might be interested in taking.
I don't know anything about the accounting units and the tax law unit. How difficult or easy you find ETC1000 will depend a lot on your mathematical/stats background.