Ask HN: Should we go with mobile or web app first?

Web.

You need a backend even if you only need an app, and adding a web UI to an API-only backend isn't that difficult.

It will also allow you to reach every device (desktop, tablet, mobile, etc) regardless of operating system or platform, where as mobile needs a single codebase per platform (no, React Native is not a silver bullet and you'll still have platform-specific quirks).

Developing a website is also easier than a mobile app, so you'll spend less time, effort and money on it than on mobile apps.