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Inline math is coming very soon! WYSIWYG unfortunately will take a while to get right, but we'll get there. Give Obsidian a try and let us know what you think! :)
You didn't ask for advice, so feel free not to take it - but you should consider using a pre-baked editing solution like Quill.js or something similar. Rich text serialization is its own problem and you shouldn't waste your development time on it :)
I was actually seriously impressed that I was able to take a huge amount of text and shove it into Obsidian and the UI stayed completely smooth the entire time. Props to the Obsidian devs for that!