Regarding multi platform targeting, have you considered something like React Native or Flutter, one code base that can run on any platform might be useful at least for the MVP stage.
Also the reason I mentioned exporting is that I’ve had to deal with a bunch of notes apps in the past where the company behind it shuts down or gets sold and then you either have to figure out how to export all those notes or risk losing them and that’s why I mostly use Obsidian on the desktop now cause even if the company behind it disappears all my notes are in my control and are in markdown format which means they can be imported into any other notes app easily.
And the 3rd party integrations API would help in getting more value out of your notes, right now I feel all my data is siloed across several apps, the articles I read and their highlights are in Pocket, my Kindle has my book highlights, my long term notes are in Obsidian, my short term notes are in Keep, my Todos are in TickTick. I am looking into a way to consolidate all the different sources of data to get synced into one platform so I have it all centralized, I saw recently that TickTick added a integration with Notion. I’m hoping to find a quick notes app that can be made to easily sync with Obsidian. Right now I tag Keep notes that I want to copy to Obsidian and then manually copy them on weekends.
Yeah my past experiences with React Native was finicky as well, but that was a few years ago, haven’t tried anything in Flutter yet, but I see your point. I thought Matrix was just a distributed chat protocol, I’ll look into it more regarding synapse