On my fire tablet I like to read Manga and use tachiyomi but afaik it doesn't seem to actually download the files of the Manga, only the Manga to read if offline I think?
Downloaded manga in Tachiyomi are saved in a folder called Tachiyomi by default and you can tell the app to save the manga in cbz archives. You can then copy these .cbz files to another drive.
I use a program called FMD (free Manga Downloader) I have it running in a container on my server and have a bunch of series watchlisted, when chapters are released it download them, converts them to CBZ and they appear in my Komga Library which I can access via web browser or Tachiyomi on all of my devices anywhere in the world.
Komga is a server app that manages comics and manga and makes them available via a web interface, has multi user management and multiple library support with read tracking and such.. think Plex for Manga.
I access it from my Phone and my Boox e-ink Tablet and if I use the webui it tracks where I've read up to between devices.