[tomsem]

Collections are completely unnecessary if you do not have more than a few dozen books on device (for me, it is generally less than a couple of dozen).

If you have hundreds or thousands on device, then you had better have a way to set them up with something like calibre.

I have something like 4500 books in calibre, 90% are in my Kindle library, downloadable in seconds. There is no system for organizing them that I am willing to spend any time maintaining, and I have no sense that it would be of any use to me even if it didn't take time. I have no issues finding something good to read quickly without such a system.