If you're planning of doing maths ext 2, physics might be better in terms of complementary subject rather than chem. There's a fair share of physics-ish topics within Mathematics Advanced+ courses. You will first encounter it in Maths Adv in the form of motion and then projectile motion in maths ext 1 and finally Mechanics which includes SHM, 3D Projectile Motion, Resisted Motion and more.

Physics gives you the opportunity to conceptualise these phenomenon early on with Kinematics and Dynamics and get used to using Newton's laws such as F=ma which you will use a lot in mechanics. This is really helpful because it saves the time required in learning the basic physics and gives you a better insight when doing questions such as drawing freebody diagrams. Physics is hard not for its maths but more of its combination of qualitative and quantitative analysis, I've enjoyed it a lot though and its a no brainer if you're going to do Ext 1 or 2 because you'd probably be capable to understand all the maths required meaning you'll have way easier time than those individuals who are coming in with subminimal math skills. You will be punished in some harder questions in dynamics early on if you're rusty on your algebra and trig (momentum, relative velocity airplane stuff etc) but other than that, the maths is pretty much year 10 level, it never progresses, you'll see the sigma and delta sign a bit but that wouldn't intimidate you.

Even then if you're not doing physics and doing maths ext 2 its not like you're at some huge disadvantage because you might not have a prior insight/understanding of the concepts because once again its more leaning towards the mathematical processes but that's the advantages I see, Physics overlaps here and there with your extension courses and you generally use the same learning approach for both subjects. Even then its not really an advantage, and you can't really apply the stuff from both courses into each other e.g suvat equations for solving motion questions in maths or differential equations in physics.

Physics is also normally assumed knowledge for engineering courses... so it might be good to consider that. Have you considered doing chem and physics together (instead of bio), they are a common combo because they pair well. But if it had to come down to one of the subjects, I would go physics because it certainly complements your math and sciences (from my experience). Both subjects are extremely interesting and I really regret not choosing chemistry over another subject, if you're doing bio just because you think it's easy and not for interest then you'll get quickly bored which won't enable you to go further.

Best luck for next year.

P.s i just saw that you meant 3U eng and Math but considering that you might lean towards MX2 throughout year 11, I'll leave the above for you so it can be in the back of your mind. Also take this with a grain of salt, let this just give you some ideas to think about when selecting your subjects - your best advisors are your teachers so consult with them.