Let me add that just as it's not enough to focus on learning Kanji, even learning compound words made out of Kanji is not enough. There are many Japanese words that are never or only rarely written with Kanji, and depending on the text there can be many sentences with no Kanji at all. I recognize 4000+ Kanji by knowing Mandarin, so I can somewhat read e.g. software manuals (many Kanji, some English loanwords) but children's books are too advanced for me.

The "777 Kanji for 90% coverage" figure is probably more relevant for illiterate native speakers. Based on a corpus of 167,281 Japanese sentences I had segmented and lemmatized some time ago, you'd need 2,685 words for 90% (up to "故郷", "birthplace", usually written "ふるさと"), 6,564 for 95% (up to "すらり", "slender, smooth"), 14,098 words for 98% (up to "鼻先", "tip of the nose") and 20,657 words for 99% (up to "恐慌", "panic"). Obviously the exact numbers depend a lot on the diversity of the corpus, so don't mistake them for fixed targets to aim for.