To control when and how to fire logic, we can use event emitters.

eventemitter.js

const fs = require('fs')
const EventEmitter = require('events')
const lazy = new EventEmitter()
const someTask = async () => true
const anotherTask = async () => true
const files = ['file1.txt', 'file2.txt', 'file3.txt']

const readFile = (file) => {
    try {
        return new Promise(async (resolve, reject) =>
            fs.readFile(file, 'utf8', (err, content) =>
            resolve(content)))
    } catch (e) {
        throw 'readFile# ' + e
    }
}

const loadFiles = (files) => {
    try {
        if (!files) return false
        return Promise.all(files.reduce((all, file) => {
            all.push(readFile(file))
            return all
        }, []))
    } catch (e) {
        throw 'loadFiles# ' + e
    }
}

const startApp = async () => {
    try {
        await someTask()
        await anotherTask()
        return lazy.on('getFiles', async () => {
            const onceReady = await loadFiles(files)
            console.log(`Additional logic and: ${JSON.stringify(onceReady, null, 2)}`)
        })
    } catch (e) {
        throw 'startApp# ' + e
    }
}

startApp()
    .then((maybeUseFiles) => {
        setTimeout(() => {
            // Whenever you feel like, trigger the aftermarket mods
            lazy.emit('getFiles')
        }, (Math.random()*1000))
    })
    .catch((meltdown) => {
        console.error(new Error('MELTDOWN DETECTED: ' + meltdown))
        process.exit(1)
    })