Interesting - and timely! I've just started working on a HN client, and the Firebase API is very barebones (N^M calls to get all comments of a given story), while the Algolia API is unfortunately _almost_ good but the comments are not sorted by popularity (making it more or less useless for a client) (of course I realized that after having coded the client for it).
Now I'm in the middle of implementing scraping of the HTML to get all comments, sorted correctly, in one call... Not fun, needless to say.
FWIW it looks like this is the approach taken by the "Hackers" iOS app [1].
I hope your modern API project will see the light of day soon-ish.
[1] = https://github.com/weiran/Hackers/blob/master/Data/Sources/D...