If you are averaging 70%+ in Extension 1, then you are capable of Extension 2 content. That doesn't necessarily mean that you'll do well, but the general rule of thumb is that if you can do Extension 2, then you should do Extension 2. So go do it. Especially since you mentioned that you enjoy it.
The only thing I can think of that proves to be a hindrance is the fact that your school doesn't regularly teach Extension 2, which is quite problematic given that even in schools where MX2 is regularly taught, teachers themselves may sometimes struggle with the questions.
But my message still stands: go for it. As long as you are getting consistently a minimum 50% raw mark, it'll still scale extraordinarily well (e.g. lower 50s raw mark in the HSC delivers an ATAR equivalence of approximately 98), and helps develops the strong mathematical background for the quantitative career you are aiming for (plus satisfying your mathematical interests).
