So for something like this I would go Mobile first and I would use this new open source technology from Google called Flutter, you can avoid JavaScript all together and go with Dart, even if you’ve never used it it’s incredily easy to learn. In my opinion is far superior technology to react native. You could get out a cross platform app with some of the basic things you describe in a few weeks.

I would use AWS Lambda or Azure functions for the entire backend for something like this. You could make it incredibly scalable and do it incredibly fast. Both AWS and Azure have great managed Document database you could use to power the features you mentioned. It has never been easier than it is right now to build some incredibly things.

Shameless plug, this is exactly what I did with a recent app I built that can take any article and convert it to Audio. Check it out https://Articulu.com got it out for iOS and Android incredibly fast.