Agreed that Calibre is a full featured management app that does it's function well but it requires readers to then use multiple apps to maintain their eBook collection.

Would be happy if you could take BookFusion for a spin. BookFusion allows you to easily upload and organize your eBooks while being able to read on iOS, Android and Desktop(via Web)

https://www.bookfusion.com/reading/cloud-library

There is also a Calibre plugin that will allow you to easily sync your Calibre eBook collection.

https://www.bookfusion.com/reading/calibre