Oh dear, you certainly are in a dilemma aren't you.
Unfortunately the pre-requisite for engineering courses is set for a reason - the level of Maths required to be successful in one of these courses is higher than others, hence why it's a prereq. Personally, I feel as though C/C+ isn't terrible - it may be you just need a bit of further assistance (e.g. ATAR Notes books, tutor etc).
If you are very keen on not doing methods, there are ways you can go about going around the initial pre-requisite. Perhaps it may just be the school environment that is affecting your ability? In any case, you can do one semester/year of a course, complete the university unit equivalent for methods (it varies depending on the uni) and then transfer over to your course... it'll take you a bit longer and financially impact you more than if you got in directly, but the option is there.
Your happiness is worth more than anything. If you're miserable and genuinely believe you cannot recover, it may be worth initiating a discussion with your careers counsellor at school to discuss next steps. I'd try it out during your early start Yr 12 (does your school do this?) this year with your new teacher (hopefully?) and see how that goes. Then if you're still not feeling it, perhaps initiate the convo.
Given your decision has multiple ramifications, it is worth seeking multiple opinions and giving everything you can a chance before making a final decision.
Thanks