Regarding multi platform targeting, have you considered something like React Native or Flutter

I have! I use Flutter in a professional setting and I do not hate it, but I am madly in love with React (not Native) for my personal projects. I tried React Native in the past and I found it a bit finicky, but it was just a “brief encounter”, so I might change my mind about it in the future. For now I would go with Electron to save time, but I do not exclude the idea to build a desktop + mobile interface in Flutter in the future. React (not Native) can also run on any platform, that should be more than enough for the MVP.

I absolutely do not want to go with Flutter for the web though, as I feel like it’s the exact opposite of what I would dream the web to be. I feel like Google built Flutter to give Android developers a tool to make apps with the excuse “oh yeah, it works on browsers too” and call it a day. Flutter basically bypasses most of the things that browsers do and in my personal opinion it re-implements everything more poorly than how a traditional web app would. Come on, they even built their own layout engine and even a rendering engine, was that really necessary? Browser extensions don’t integrate well with Flutter apps, and the debugging experience is subpar. I really like Flutter for Android and desktop apps, and I might even say it’s easier/simpler than Android Studio (Kotlin), but personally I feel like it’s just a big dirty hack in the web context.

Okay, I just realized that my reply sounds a lot like a rant. My apologies. Please consider this as my current view on these tools, it might change in the future. It’s not a criticism.

the company behind it shuts down or gets sold

That’s an important concern. Well, the source code is AGPL 3.0, so there is no risk of it disappearing. Also, this would be my hobby project and I absolutely do not expect it to ever become anything commercial. I do not even plan to accept donations or anything like that. Basically it’s something that I need, and since it doesn’t exist I am considering building it myself.

There is a big probability that I might stop working on it either because I find it “good enough” for my needs or because I won’t be able to work on it anymore, but I would say I’ll try to keep it as simple as possible, so it should be relatively easy to maintain. Also, relying on Element (Matrix) maintain the server code would mean I just need to maintain the client, which should cut the work down by 50%. I strongly hope Matrix will never shut down or get sold.

In any case, okay! Let there be the possibility to export data in a common format! (Preferably Markdown)

I feel all my data is siloed across several apps

Matrix supports a way to integrate with third-party data sources, but I haven’t read too much into it as it was not in my goals. Perhaps a Synapse (Matrix) plug-in might allow to integrate notes with third-party sources without having to make any changes to the client code. I might investigate this possibility in a later stage.