Loosen your stem pinch bolts so the handlebars move freely, independent of the front wheel. (This is essential, and leave it loose until later)
Remove the SpeedX top cap/button and let it dangle....take care as there are a couple of soldered wires.
Ensure your headset bearings at top and bottom of steerer tube are aligned/flush. Also check the spacers and top cover of bearings is aligned with the head tube as I've seen lots of cases where there is a small gap judging by people's photos and this isn't correct - it could be misalignment, it could be SpeedX components that just don't sit flush..... start tightening the top cap bolt that goes into the compression nut, this puts pre-load into the headset bearings.....but before that actually happens the tightening of that screw will pull the forks up into the steerer and remove the play that you have and the gap that is showing in your photo. Lift the front wheel off the floor and ensure the handlebars turn freely, if you feel any resistance or grating (bearings under compression) you've gone too far. There is no recommended torque here, you have to do it by feel and all headsets are slightly different. You can also put the front brake on as you've done before and rock the bike forward/backward to check if the play is eliminated too.
If at any point when working on the pre-load of the bearings you feel the compression nut/wedge slip or spin within the steerer tube (unlikely) you will need to tighten that up. Again, there is no recommended torque and you only need to ensure that it doesn't slip when pre-loading. You're probably looking at approx 3-4nM however be aware the last thing you want to do is go too tight and damage the carbon steerer tube so 'just enough' is perfect.
When you're happy with the pre-load, check the spacers and stem are seated correctly and are as as compact as possible on the stack and tighten up the stem pinch bolts to the recommended torque. This is the only point at which you are securing the top/bottom bearings into position and the fork steerer into its permanent position. Once stem is secure, the compression nut/wedge has done it's job and can be backed off if required, the only job it serves to do now is to keep the 'inner top cap' on a SpeedX bike in place and can be hand-tight.
Insert the SpeedX top cap/button and secure. Double-check there is no movement on the spacers/headset and forks.