New: Qalculate! (Sep 29, 2022), Platform 22.0.1 (Jul 21, 2022)
450+ real apps (49GB), 1.06 billion downloads, Please donate.
This app would be nice to add.
At least, you may try the already present ones: KeepNote & Cherrytree. There is also another one → https://treeline.bellz.org/
Illusions are an infinite paradise.