hmm, I solved the problem differently. Since the data I need relies on a user interaction that's certain to happen after a small buffer, I attached my need to fetch data to the user clicking the reply button, when it's sure to have been loaded in the store. For reference, here are the first few lines of the component:
import app from 'flarum/app';
import Component from 'flarum/Component';
export default class ByTagsComposer extends Component {
init() {
app.store;
console.log(app.store); // outputs the app.store as expected
const allTags = app.store.all('tags');
console.log(allTags);
}