Conversationally, in a beer over a barbie kind of way, no offence intended,
Dale-Chell lost me at "words that 80% of fourth-graders in American schools knew in 1948 (revised 1984)".
A cookie in 1984 is not the cookie of the 2020s web savvy teenager.
This particular article from the UK shares many of my own feelings about automated readability indexes:
https://www.effortmark.co.uk/readability-formulas-seven-reas...
I still admire your goal here, I'm simply suggesting to rely on a modicum of actual human feedback over automation where possible.