I long ago (40 years) settled on helmet, then eyeglass-mounted mirrors. Mine gives the same view as a 23cm/9in hand mirror held at arm's length and I can sweep two traffic lanes with a slight turn of my head. Head-mounted mirrors do take some getting used to as eye dominance comes into play. I either see "through" mine or "into" it depending (but I also use monovision contact lens vision correction, so I'm pretty used to playing with eye dominance; uncorrected eye is for near vision, corrected is for distance).
Critical to making them work is getting the right distance from your eye. It takes a bit of playing at first, but once you find it -- bliss!
I was concerned about injury in a crash, but each time I've gone down, the mirror came off as it was supposed to. I think it helps I'm wearing wraparound goggles with polycarbonate lenses.
After trying every model of helmet and eyeglass mirror I knew of on the market over the years, I settled on this one for the Bolle Edge II riding glasses I use when riding all my bikes:
https://www.amazon.com/Bike-Peddler-Cycling-Eyeglass-Mirror/dp/B001VTQNVOI have a number of bikes <mumblefifteen>, so it helps to have one mirror to <ahem> rule them all and go with me no matter what I ride. Since my position is the same on all the bikes, I don't even need to adjust it when switching mounts, but it is easy to do if needed and no concerns about breakage if the bike goes down.
Vive la différence!
Looking forward to see what best suits Henry, Ian. Keep him happy and you'll be happy as well.
All the best,
Dan.