I have no comment on merits of Akido. I have not used fbreader seriously but it looks OK.
You can put .epub files in
Kindle\Internal storage\Documents
as shown on the PC when the kindle is connected by USB, and fbreader finds them.
The native Kindle files seem to be there too but naturally enough (sorry for the cynicism:-) the names are mangled.
On the kindle the ebook info says the folder is called
mnt/sdcard/documents/
This appears to be the same place.
I have not tried folders of books, this may or may not work.
You could also consider using Calibre to covert all the files to .mobi but then they would have to be emailed to the kindle in batches.
In FBReader Bookshelf View is pay version only but view by Title/Author works.
Good luck.
[edit]
Kindle\Internal storage\Books also works. Folders of books don't get seem to get recognised automatically but you can browse the Books folder for folders and open books therein. Once opened the book appears in the recent list but still doesn't appear in the normal list of books.
The "books" folder is configurable. Possibly more than one.
Last edited by jimjim1; 14th Feb 2020 at 11:13.