A few folks recommended Quarto to me when I asked about current recommendations recently. My main goal is physical books and that doesn't seem to be a first class citizen anywhere. Looking through Quarto, I can't find reference to markdown syntax for index entries (which indicates that they don't support it.)

Also, I'm heavily biased, but based on my experience I will need to hack whatever tool and I'm not really interested in hacking JS/TS. (My whole reason for getting rid of my rst based tooling is to simplify and sticking in Python simplifies my life, coding in JS/TS not so much.)