[Tarana]

I have too many ebooks to keep on my ereader. Even reached the limit just in my Amazon library (only 1,700 of 1,900 fit on my Voyage). I keep my books in Calibre, convert everything to azw3. I use the plugboard to add series info and typically have 50-200 books on any one ereader (I have a bunch of them). Then I sideload a couple hundred onto ereader.

I have never been able to get Collections to work well for me.

Try the Calibre method especially if you buy a lot of books. See how it works for you.

Edited to add: FWIW, I do not email ebooks to my kindle because it becomes a mobi file. mobi doesn't provide the font options available in azw3 which has become rather crucial for me.