The problem with the send to Kindle option is email attachment size limit. For books it’s fine but manga and comics are usually too big.

My Kindle is old enough that I was able to jailbreak it and install KOReader so I can just download from my Calibre server directly via OPDS. Otherwise if it’s too big for email you’ll have to do it over USB I think.

Great tips. I hunted 2.5" HDDs when I was doing my build but they seem to be on their way out, being meant for laptops which are now largely just using SSDs :(

TIL! Sorry about that, I never actually tried adding ebooks to it so I had no idea

Sounds like you want Calibre + Calibre-Web! Web has a nice frontend that lets you send to Kindle, or download or just read right there. There are definitely apps that let you link to that library and read the books that way. If you have a different ebook that supports KOReader, even better. You can add your calibre Web

Someone below mentioned Audiobookshelf which is great but it for audiobooks.

Bitwarden now has their own secrets manager!

https://bitwarden.com/products/secrets-manager/

Ignore the enterpriseyness of that link. Once you log in normally just make an org and you should see the option to use secrets manager. It’s free for up to two orgs.

I haven’t tried this, but a combo of

YTDLP subscribed to a playlist of your choice, downloading to a directory which you can mount to Jellyfin and make a library should work.