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.
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 :(