Hi Vanessa,
There seem to be a few things working against. Firstly as you said your study scores which are a barrier to entry for working for tuition companies. I should probably note the usual: High study scores doesn't inherently make someone a good teacher and "low' study scores doesn't mean someone won't be a good teacher. Anyway, your 40+ scores are still definitely great and should not be a problem for tutoring. I had a friend who only scored in the mid-30s for a subject (somewhat bombed the subject) but still ended up being quite a successful tutor in that subject directly after VCE.
Another thing working against you is the time of year is the holidays nearing the New Year so I would think most people are not actively looking for tutors at this moment in time - possibly once 2020 turns around I think a lot of people will being searching. Even then the deeper you get into 2020 the more students will be actively searching for tutoring. It is common for a student to think they are ok at first and after the first sac realise they may need additional help (friends of mine that tutor say they get a heap of requests in this time period around March-May.
As for where you can advertise there are a few places.
1. ATAR Notes I see you have posted an Ad over there which you are allowed to bump every 30 days. For a kind of indirect bumping, I would suggest showcasing your tutoring/teaching skills on ATAR Notes. You can do this by answering questions on threads relating to your subject. You can include a link to your tutoring Ad within your signature. This means every time you post you will be advertising. This has many benefits as you have a continuous audience to your Ad along with the fact that most students on ATAR Notes are very dedicated.
2.Gumtree
3.HighSchoolTutorsI can't speak too much on these as I haven't used it much but they seem pretty popular for parents/students when they are looking for tutors.
btw I saw your Ad and your trial lesson is an excellent idea
Hope this helps, good luck