I am not a super elite player or anything, but from my perspective of playing SoD priest healer, there is no reason to pick Holy spec over discipline. Discipline, coupled with runes, makes holy spec obsolete. It offers more utility and comparable (if not superior healing) to Holy spec.
Holy spec had issues in original classic, many priests would never talent Lightwell, simply because people just wouldn’t click it, and the talent points were better used obtaining Divine Spirit.
So to remedy the Holy spec issue, my suggestion would be to make a rune that makes Lightwell worth talenting and using. Something along the lines of ‘slightly reduces the healing by x and increases the mana cost by x, but it applies the heal to anyone in a x yard radius’.
In doing so, you also make the Spirit of the Redeemer rune relevant, as you will have to talent it to get to Lightwell in the holy tree anyway, so would give dispersion much needed competition in that rune slot.
On that note, I think Pain Suppression needs to be moved to a different slot. It is a great rune but it is unfortunately competing with much better options. The mana return of dispersion makes picking PS a waste. To top it all off, it is now also competing with Void Plague. Maybe move PS to the chest slot?
Those are my thoughts anyway, for what they are worth.