I have a few more suggestions for you.
I'm in South Australia, so I can't guarantee that university application and selection works in exactly the same way in Sydney, but I'm confident that this will still be helpful information for you.
Since you haven't finished Year 12, you won't be able to apply to a university course in the same way as school leavers, because you don't have any grades for the university to judge you by. Which leaves you two options. You can either sit a Mature Age Entry Test (that's what it's called in SA) once you are over 21, which you are. It is possible to do a preparation course for this test before you sit it.
Another option is to do a bridging or foundation course, which is another pathway into uni for students who haven't finished Year 12.
But since you mentioned becoming an ESL teacher, I actually have another suggestion for you that doesn't involve uni. The most recognised international qualification for ESL teaching is actually the CELTA (Cambridge Certificate of English Language Teaching to Adults). Language schools and private colleges offer this course, usually as a month long intensive course or a semester long part-time course. The cost is between $2000 and $3000, so it is expensive, but obviously cheaper than uni.
I work as an ESL tutor so if you have any questions about it, please contact me. I hope I can be of some help.
Good luck with your decision making! Remember, don't rush in to anything. You will find what is the right course for you.