I already had to spend the time organizing all my books. That doesn't really matter if they are in the cloud or on the device I have 48 collections. All my books are in those collections, many in more than one. So that is the same no matter how many of those books are actually on the device. I just like to have a nice selection on it to pick from. Since my kindles never had the issue with having to do a total reset, I don't worry about that. If it ever happens, I deal with it then. No point in worrying about something that has never been an issue. For 9 years I haven't had an issue. So odds are in my favor on that.
300 or so books from my large catalog is a pretty nice number on my device. I often jump into books and read the about the book to see what the book was about. I am a big mood reader and I can't barely remember what the plot was, or blurb of all the books I buy. I don't want to have to download those each and every time I pick the next book to read.
I don't know if my devices would slow down if I put all my books on it. Over 3000. But with 300 or so, I have no issues. At some point that will be more like 500 as I keep adding books to it as I use a kindle as a primary reading device.