Extremely positive, they were lovely. However, although the VCE content is exactly the same to mainstream, the delivery really is different. As Year 12 has enough pressure and obligations, I think it's worth taking term 4 to adjust to new factors that your competitors have had years to adjust to. Not all factors may apply to you, but variables like working off someone else's 9 AM - 3:15 PM schedule and not your own, having to keep up with the teacher in the in-class room environment instead of having the luxury of going through the handbook at your own pace, needing to take all the notes then and there instead of again just returning to the handbook, familiarizing yourself with groupwork. Many things.
There are outweighing advantages of mainstream high school as well. You would pick up things there that you just can't when you don't meet your teacher, coordinators, career advisors or classmates. I ended up picking the wrong degree for my personal situation because I had no idea that UoM was one of the best choices for neuroscience; I chose a university that just seemed to have a nice campus and was 10 minutes away ahaha. So I was forced to just scrap into a first year midyear transfer when the ATAR req was 95 instead of 91 @_@ and I lost my scholarship. After Year 12 I learnt from people at university all this stuff about VCE scaling, appropriate study techniques (I used to rote-learn everything, which DECV kind of forces you into as it's mainly just reading text online), the fact that the same or similar questions come up from previous practice exams (I didn't do a single practice exam in VCE), all the little things that add up in a big way. Apparently many schools have SACs in Year 11 as well to prepare their students, which DECV doesn't. It will also be an advantage to familarize yourself with classmates this year, as you can form study groups over the holidays if you wanted to. Learning through study groups is also usually more effective than just reading off the textbook, and I had no idea until university.
Again if $600 feels disproportionate, it wouldn't be worth transferring in term 4 because while the differences are there, if you're determined then your goals are achievable regardless of which decision you choose. And you have mentioned that you would be completing DECV's work regardless of where you attended, after all. Hiring decent tutors for half the price was a good suggestion.