I’m using a combination of:

• The Boox Palma reader, though they have larger tablets if you prefer. I’m not sure about the others, but the Palma runs Android with the Play Store.
• Kavita to host my ebooks online.
• FolderSync with SFTP to sync all of my books ahead of time to my SD card.
• Moon Reader to add my Kavita server’s OPDS feed as an online catalog if I need to grab something manually.