Ok for the start why did you delete serato ?
If you have good hard drive space there was no need do that.
I own ddj sz n myself been running rekordbox dj fine but if your running windows 10 the driver for the sz is not supported by windows rekordbox dj is so that could be a issue.
But I am running windows 10 myself I did come across a issue when I first try it.
Rekordbox it's self wouldn't recognise the sz plug in.
So what u did was reinstall rekordbox and the driver now it works fine part from when I want to use rekordbox on its own it won't work without sz plug in for some strange reason.
Might been luck it's working for me but that's how it happened for me hope this helps