The single thing which improves the visibility of cyclists to drivers is the cyclist's road position. You have to ride where drivers are looking, particularly at junctions - which means you have to stay out of the gutter and take the lane when you have to, especially passing side roads.
The efectiveness of helmets and hi-viz is hugely overstated.
Road position is just stating the obvious. Surely most of us do that already? I also seek out roads with less traffic where possible. Cycling in areas with good infrastructure for cyclists means using cycle lanes too, which can help reduce risk - another obvious one.
In addition to road position, I wear clothing that stands out from the blur - motorists often do not recognise anything else but what they are specifically looking for: other motor vehicles. I believe it helps to try to highlight myself and I do this from personal experience as a motorist who notices the difference when driving between a dark-attired cyclist and a brighter-attired one.
It's a personal risk assessment - you do what you believe works best for you, of course, however I disagree with your assertion that helmet's and hi-viz are overstated - happy to leave it at that since I'm not trying to convert, only state my own views for anyone else reading who may be seeking other's experiences.
I would likely have suffered brain damage without a helmet in my last racing accident but then I typically cycle at much greater speeds than most.
Cycling around Amsterdam when I lived there I never wore a helmet unless training; so slow commutes, leisurely rides around the city etc I didn't bother, admittedly. Most don't, actually, albeit you do see a lot of children wearing them. The speeds are much lower as is the risk of being involved in a car accident. Still, I've witnessed a tourist with a nasty gash to her forehead from simply falling over.
There is a notable perception amongst some cyclists, I have found in my travels, that some believe they are 'cycling God's' when it comes to "road craft" and smugly brush off the need for any other risk-reducing measures in favour of their enormous skill levels. I know cyclists who had better road craft and bike handling skills than any of them who sadly aren't with us today thanks to motorists who weren't paying attention. No amount of "road craft" will save you if the other party to the accident doesn't pay you any attention - even if you are nearly directly in front of them.
Anyway, happy cycling to all - one thing we can agree on is our joy for cycling!