"Enhances the next spell cast"
is misleading because Sharpcast doesn't apply to Flare, Despair, Foul, or Xenoglossy.
"Will trigger their additional effect"
and similar wordings are, at best, uninformative. Casting the affected Fire and Thunder spells always triggers their additional effects; you don't need Sharpcast for that. The problem is that for, say, Fire (Fire 1), the additional effect is technically "the game will roll some dice for you." Sharpcast doesn't trigger that dice roll; Sharpcast straight-up grants you Firestarter.

This is all why Sharpcast's tooltip is worded the way it is. If anything, I think you can simplify it as follows:
"Ensures the next Scathe, Fire, Paradox, or Thunder spell cast will, for the first hit, trigger Scathe's additional effect, Firestarter, or Thundercloud."
Trying to account for the DoT applied by the Thunder spells is more clunky than clarifying.