Have you considered a torrent based solution?

Setup a torrenting server hosted online somewhere with fast speeds (e.g. Netherlands), sync it with gdrive, create a torrent for your books, and “download” your books to the toreenting machine.

This approach won’t help with upload speed, but it should help with reliability/continuity. As long as you seed from your home computer, your books will eventually make it into your gdrive.

Although with this setup it might be easier to just torrent the ebooks to the online server from sources other than yourself.