Do you use the rules that have a set DC associated with the Wand? For example, Wand of Fireballs is fixed as a DC 15 save which makes it seem like the Wand is somehow involved in the casting more.
Since the item specifically states a DC, it's always a 15 (although I've debated changing that). Everything else is applied as if the PC were casting the spell in my campaign. So a high level evoker gets to add their int mod to damage, etc.
On the other hand a Wand of Fear is just "use an action" to get the result based on how many charges you use, so the PC is not casting the spell. Nothing the caster has affects it.