Well it is either Crusade or Divine Purpose. Both are better paired with JV. Add 2,5/5 seconds to its duration with relics and Crusade probably pulls ahead. Honestly though you will be switching talents a lot as ret anyway so I'd expect the choice to change per encounter.
Ofc if you get the right legendaries Crusade will pull ahead very fast.
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That is a very weird way to see this.
The difference in going for Ashes to Ashes first or last is essentially delaying your third gold trait. You are exchanging Embrace the Light for Protector of the Ashen Blade (which is better in group environments), Highlord's Judgement for Righteous Blade and Divine Tempest for Ashes to Ashes. AtA is better than Divine Tempest and Righteous Blade largely covers the damage buff from Divine Tempest.
You were looking at it as if you'd only have 13 traits when the raids go up. You are more likely to have 20-21 traits by then (and a second Golden trait).
These are the paths at 21 traits:
Ashes to Ashes then Echo of the Highlord
Ashes to Ashes then Divine Tempest
Echo of the Highlord then Divine Tempest
JVs damage per holy power certainly is much worse than TV is. But again, noone suggested you'd use it for dps. You would use it with Divine Purpose procs (where it does more damage even than a fully buffed TV, even if by a slight margin) or to stack Crusader in 3 globals instead of 5. Considering the alternatives in its row are not a dps gain, unless you needed the defensive cooldown from Eye for an Eye JV would be prefered. And again, many people will also be running Mythic+. Trash, raid boss adds and dungeon boss adds can be stunned and JV on a stunned mob is simply the best use of Holy Power.