Magic Resistance and Destructive Wrath

Currently hit a debate within our campaign in regards to gaining advantage on a Saving Throw from Magic Resistance when our Tempest Cleric can use their Channel Divinity for Destructive Wrath.

I have already checked these two posts: How do I know if an ability is magical? and When does a Tempest cleric have to choose to use Destructive Wrath?.

If the Cleric uses his Wrath of the Storm ability to deal Thunder/Lightning damage the target has to make a Dex Save. As per the Sage Advice this is not magical and therefore isn’t made at advantage by a creature with Magic Resistance.

If the Destructive Wrath is used as a Channel Divinity it gains the wording "fuel magical effects" and would therefore become magical for the purpose of Magic Resistance, however the most upvoted post on Destructive Wrath puts the damage being rolled after the saving throw and thus applied after the saving throw is made.

My question is what would be the best way to apply the rules in this example?

Would you apply the Destructive Wrath after the saving throw has been made thus detrimenting the creature, or would you apply the Destructive Wrath before the saving throw thus (potentially) detrimenting the player?