I wonder if part of your hesitation is not wanting to start something that it's not clear you can sustain, financially. My own experience is that even a few years of a school experience that is a great fit is worth it. Not as good maybe as all 12 years, but some is definitely more beneficial than some, and the more the better. I went to Montessori through K, then a few years on not-great-academically public schools, then a fantastic, wonderful private school that was so good for me, socially and academically both. Then a public school that was supposed to be great academically, but wasn't a great fit for me. I am so, so immensely glad and thankful for the years I had at my good school. The things I learned there are the things that have stuck with me, and the sense that I could be a good learner stayed with me through years at the less-good places. I do think there is tremendous value in building the memories of what it feels like to be working in that sweet spot academically, and of being in a learning environment that feels right, even if it's not sustainable. Especially if not. I think those memories are a great gift to carry through leaner years.
But it's so hard to judge other people's finances! Being up nights worrying, not knowing how to pay the $5 for a class project... at some point there's a tipping point, and it's so hard to plan in advance where the tipping point is, because how can you predict what might come up? Can you work as a tutor if you need to get some extra cash at some point?