I'll add some more pointless two cents. I use, and love, Obsidian for the following reasons. I'm a research engineer, and it supports latex equations, markdown formatting, pasting images from the clipboard, and syntax highlighting. It is also beautiful from the get go (as is your app); the line spacing is just-right, the colour scheme is pleasant, the editor and preview mode typefaces are both delightful. But, I do use it as a full-fledged note taking app.
In your shoes (and contrary to the spirit of the thread, I admit), I would definitely spend time getting to know to whom you're targetting this app. Perhaps yourself, which is fair. But, for instance, it wouldn't work for someone like me. Still, I like what you've done here and I wish you all the best!!
The only things on your list that I hadn’t considered was latex equations and whether I should offer different design customisations of the cards.
I’m glad you picked up on the index card size default. I will offer both full-page notes as well, but there’s another reason why I want them as card sized—so you can lay them out on a virtual workspace to help answer questions from your slip box.
Honestly, there’s too many ideas that I can’t do everything. But it’s sure fun working on it and gradually making it better each release.
Early (no signup/in browser storage) version is live at https://app.flowtelic.com.
A video showing how to use it is here: https://youtu.be/Zo9hIuffz_0
I’m documenting as I build this over on Twitter at https://twitter.com/Martin_Adams