Thryx, the Sudden Storm

• WOTC Official Reminder Textofficial rules
  Flash — You may cast this spell any time you could cast an instant.

• WWizards of the Coast2020-01-24
  Official Ruling
  For spells with in their mana costs, use the value chosen for X to determine the spell's mana value.
• WWizards of the Coast2020-01-24
  Official Ruling
  A spell or ability that counters spells can still target spells you control with mana value 5 or greater. When that spell or ability resolves, your spell won't be countered, but any additional effects of the countering spell or ability will still happen.
• WWizards of the Coast2020-01-24
  Official Ruling
  To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions (such as that of Thryx). The mana value of the spell remains unchanged, no matter what the total cost to cast it was.
• WWizards of the Coast2020-01-24
  Official Ruling
  Players can't attempt to remove Thryx to raise the cost of a spell you wish to cast, but they can remove it to allow their spells and abilities to counter your spells.
• WWizards of the Coast2020-01-24
  Official Ruling
  Thryx's last ability doesn't apply to itself while it's a spell.
• WWizards of the Coast2020-01-24
  Official Ruling
  If Thryx enters the battlefield while a spell with mana value 5 or greater that you cast is on the stack, that spell can't be countered, even though you don't get the discount.

Rulings via Scryfall