Is there a reason you think this is adhd and not autism?
I understand you've documented a lot of hyperactivity and impulsivity but these can be present in autism, it's the mechanics around it that would infer one or the other or even both.
Autistic children can be very self-led, and unable to follow adult led instructions reliably. There's a whole therapy programme by Gina Davies called attention autism for this reason.
Flicking lights, taking medical equipment, being a whirling dervish can all be investigating cause and effect.
Being hyperactive like Taz the Tasmanian devil can be vestibular and proprioceptive sensory seeking behaviours.
Lack of danger awareness, eloping in public, can be attributed to the social and communication criteria for autism
The meltdowns, well it depends on triggers but can be dysregulation due to a change in (perceived) plans, schedules or routines even if those are self imposed by your child and not in line with how your day goes, sensory overstimulation, sensory understimulation, a lack of autonomy, and just developmentally being delayed in the emotional regulation department.
I would honestly consider the possibility of autism. I feel like there's probably more to it but from what you've said in your post, I feel he meets the triad/dyad of impairments.
On top of that he can be assessed for autism sooner, and it sounds like he will need an EHCP during his education to get him the best possible start, and you don't need a diagnosis for an EHCP but you do need a lot of paper evidence which comes from assessments.
I'm also wary of health visitors and unspecialised professionals saying ADHD at such a young age. Not because it might not be true but because some use it as autism-lite and it means you aren't getting the right support at the right time as you've been sent barking up a very tall, different kind of tree.