In the OP's own words:
It took a few seconds for the reality to sink in ...
That might be presented by the defence as
.. and for my mind to fill in the gaps in what I had witnessed.
That's not intended as a criticism of the OP, just a reminder that a victim's recollection may be incomplete.
The OP's evidence is corroborrated by the camera footage in terms of when and where, but there's not much shown of the rider's behaviour, other than a brief view of his raised arm. IMO, it wouldn't add much to a prosecution for assault, unless the suspect denied the time date etc.
The biggy, of course is that the OP was able to stop the car driver immediately behind and to obtain their details with a view to their being a witness. In a case like this, an independent witness with a front-row seat is the make or break. AFAIK, We know nothing about this part of the evidence (by coincidence, I've had a reminder today from Specsavers that they've not seen me for over three years )
Then, there's the record of what was said in the police interview of the defendant. This is summarised, second hand as a claim of self-defence but it shouldn't stop there, but rather it should be probed, especially if the independent witness's version does not support it. If there's to be a formal caution, the interview must have reached a voluntary admission.
Beyond that, it's important in assault cases to be wary of dwelling on what might have happened when it didn't. It's impossible to attempt an assault, since what might normally amount to an attempt eg an unsuccessful attempt to hit somebody amounts to the full offence. ie Although "assault" is popularly taken to mean the landing of the blow, that's legally the battery. Then, there's a range of charges largely based on the gravity of the injuries caused. Although the Offences Against the Person Act, 1861 is overdue for reform, ss 42 and 47 were repealed and replaced by s.39 Criminal Justice Act 1988. The point here is that relatively recently, Parliament made common assault a purely summary offence, without the option to take the case to the Crown Court where the max would be 5 years imprisonment if the circumstances were serious even though the assault itself could not be proved to have caused serious injury.