Besides vitamin D (much of which comes from exposure to natural sunlight) being necessary to absorb calcium(true of all living things), a good ratio of calcium/phosphorous of 2:1 is important too. Excessive calcium or insufficient phosphorous can also lead to soft eggs. So if one is still...
Any light bright enough to barely read a newspaper by at roost height will work. Most of the year I use a compact fluorescent because they last and don't use much energy. In the dead of winter they don't work as well so I switch to a 40 watt incandescent.
Other than Vitamin D, a balance...
I've had soft eggs but never a bird that laid only soft eggs so I'm not sure. Vitamin D is important in the absorption of calcium.
Sounds like you're doing good things.
If you give treats or scratch I would cut that out entirely and only give layer feed, ACV water and either oyster or egg shell...