Istbor
Dances with Gnolls
You can up-armor the skeletons as well, like in Walking Dead where they poured molten metal on the zombies to make them harder to kill. A skeleton with metal bones must be an AC 18 or such and I would take away the vulnerable to bludgeoning ability.
Mithral?
Yeah, I actually had a similar thought. That over time the player can fit the raised champion skeleton with different weapons or some better armor, if found (what does the skeleton care about that damaged chainmail?). Which is part of the reason I wanted to come here and get ideas as well, because I am worried somewhat about the power of this spell. I want to make sure it doesn't overshadow the fighter in the group.
I have so far, tweaked the stats on the Thug, from the DMG, and am using that as the base stats for the raised skeleton. Also changed it to a 10 minute cast time, as you are calling up a more powerful undead.