My Calibre setup
This is just a quick write up of my current Calibre setup. I mainly read on a Kindle Voyage but also occasionally use the Books app on my iPhone. When the Kindle finally dies I'll most likely get a Kobo.
I have Calibre installed on two computers: a Windows desktop (that is currently offline) and a Mac. My Calibre Library folder is on OneDrive. This allows me to access my books on all my devices. I never have sync issues with this setup, but I have read that some people experience issues if they try to do it with Google Drive.
When I buy an ebook, either from Amazon or Kobo, I use the NoDRM plugin so that I can convert it to the other format — so I usually have an AZW3 and an EPUB version. This allows me to read any book on either the Kindle or iPhone.