This is what I do with my base model Kindle. I have its email address saved as a contact so it is super easy to send it ePubs (or PDFs!) from my phone.
I'm sure you're aware that amazon can discontinue this at any moment. My ebook reader just reads an sd card or I can use a usb cable to transfer files. That's harder to remotely kill.
You can use usb cable to transfer to kindle too
I'm aware. If they discontinue it then I'll just stick to Kindle books. That's all I'm expecting from this device anyway. For everything else I have an iPad Pro.