selectives are good but not good, if that makes any sense - some selectives I hear just assume their students have tutoring, even at the highest level, and so teach in a manner that is catered to that. it's also a lot more rote learning. but, it gets results, and the competitive environment does help some students, so they're decent. whether a selective is good or not I feel entirely depends on the student and what kind of learning environment they need. but it isn't the school itself that is getting them those ridiculously high schools, it's the fact the students are cherry-picked to be the best, and the school just tries to maintain that (in my experience).
private schools can be very hit or miss. some of them are quite good at bringing the average up, with a handful of teachers that do their utmost to assist the entire cohort. I kinda agree that top 50 private, you have to be good at something lest you get left behind, but at lower levels (50-150) privates can be quite good.
what my mum used to do was go to myschools and look at the NAPLAN results for a cohort in year 7 (e.g. 2023) and then in year 9 (2025 would be the same cohort) and see if they improved. for example, baulko's writing average actually decreased between these two years for the same cohort, demonstrating that in this area, they weren't improving as much as the rest of the state. (not attacking baulko, just citing an example I remember - though I don't remember which year.) yes, they were better than other schools, but there was no actual improvement in the cohort itself. so, this might be a method to see how holistically helpful a school is.
this is just my take!