mixing in rekordbox

Rekordbox Export mode was designed to prepare your tracks and check if tracks are compatible with each other. And then export to use on hardware like the CDJs. Nothing more than that.

If you want to do more than that you will need to get Rekordbox Performance Mode either with a subscription or hardware that unlocks it. 
Then you are able to use EQ and other features. Same for having 4 decks. 

There are other DJ software that allow you to do what you want like Virtual DJ.
What the base version of Rekordbox (Export Mode) can do is pretty well documented and also described in the manual.

Ah ok, thanks for that. I have xdj 1000 mk's, would that make me eligible for the performance mode?

The rekordbox website says:

"

WHAT'S NEW IN THE FREE PLAN?

Performance mode via PC/Mac

As well as free use of Export mode (for transferring tracks to USB drives and playing them on CDJs and XDJs), you now get access to Performance mode when you control it with your computer. This means you can try out mixes, cues, loops, FX, and more in a professional virtual DJing environment. You can even mix with tracks from Beatport LINK and SoundCloud Go+ if you have subscriptions to those streaming services."

Does this mean I have to have my computer connected to a hardware device in order to use it? Just doesn't seem very clear on the website to be honest. My version looks so much more basic compared to what they're advertising on the website even for the free version. Nowhere can I see a picture of rekordbox which looks similar to the one I am using (6.5.2)

No - the XDJs can control performance mode, but per the hardware list here, you'd only be able to play in EXPORT mode.