Hey! Completely understandable that you aren't sure, cause really, who is? I'm a first year economics student at USYD, and my brother is a mech engo/advanced science student at UNSW, so I can give a bit of insight into what both can offer in terms of uni and career-wise (tho, of course, I know the econ one a lot better).
If you like stem/science for things like maths but are tempted by economics? Good news! Economics will have a lot of maths. A lot. Depending on which way you choose to focus on it in uni, like econometrics or economics, financial economics, environmental, etc, chances are you will come across some pretty high level math, in a some ways similar to stuff you would find in stem related degrees. What's different is the application of course, but in the end, you do get a lot of the underlying skills in mathematics that you would in stem or economics. Economics is a little bit less of a guarantee for the higher level maths, particular as that won't usually be in your first year, but you can still tailor you degree to have more of a maths focus.
In my opinion, economics is a really broad and interesting field, that if you can learn how to apply properly, can open you up to a lot of career paths. I'm currently a cadet in equities research and being able to see the stuff I'm learning in classes happen live is amazing. At the same time, with just classes, you can also see all the different areas and is so versatile.
That being said, I think it is important to remember that this decision and what you study now is not the be all and end all. You can always change your study down the track or just apply your knowledge in a different field. For example, two people I know hold doctorates in their current job, but their doctorates are in a completely different area to what they are doing now, but they are still using that knowledge they have from that qualification. There is no one degree that can guarantee you better job prospects than any other, it's just a matter of how you apply yourself. Trust whatever decision you make, burn your bridges, and don't look back. Best of luck! You've got this