A lot to unpack here but I don’t think computers have anything to do with it
Discussion
I've always been wary of getting used to glasses because they literally (for myopia) reduce the tension required to focus the lens. So, not a permanent issue, but one that takes time to recover from. Use it or Lose it as they say. Antifragile systems need to be worked to increase capability, and muscles are exactly this. A little strain makes them grow more fibres and that increases the lens ability to stretch.
And conversely, if the diet or some other factor causes the lens to be harder, this will also have a similar effect as not working the muscle, but this will be difficult to reverse.
IMO, for myopia, glasses should be a second line not first line treatment, because the lens naturally hardens (and yellows) with age. Nutrients and exercises can address the lens flexibility. And the other thing is that the lenses themselves filter some types of light, which may be another factor.
There's a lot of moving parts in the vision system. it would be hard to positively narrow down reasons for the japanese higher than average use of corrective lenses for short sightedness.