Unless that rock is the only +5 weapon in the multiverse.
When examining how high an attack roll could go in 5E, I stipulated a +3 rock, having read that +3 was top end in 5E. Attack rolls can go much, much higher than AC in 5E. Vrax, just by level 20 and the Belt, hits AC 23 even if she rolls a 2, and that jumps to AC 26 with the +3 rock. Which is to say, she doesn't need the rock, nor an attack roll higher than 2, to hit the Tarrasque, unless it has Cover or some such.
When examining how high a stat check can go, such as throwing purely for distance, the +3 enchantment on the rock is IMO irrelevant.
However, there ARE times to roll a stat check in combat, such as Grappling; a level 20 character with the Belt and Expertise in Athletics gets +28 to establish Grappled, then +28 to Shove the target Prone. If Vrax with the Belt rolls a 2 on the check to Grapple/Shove, and her target rolls 19 on the opposing roll, then Vrax still succeeds unless the target has at least +10 to their Athletics or Acrobatics check. *That* is what I'm examining: extreme, edge-of-the-envelope cases in Bounded Accuracy. If Vrax is fighting a low-level foe, then the Athletics check is almost a formality. Unless she rolls 1 or the foe rolls 20, she wins the contested check.
Moral of the story: if you are low-level, don't wrestle with a level 20 character with a Legendary magic item.
Are Guidance and Inspiration really the only two factors which can add to the d20 roll, rather than grant Advantage, on a stat check?
If Vrax is a Lore Bard, perhaps with Guidance as a Magical Secret, then she can take Expertise on Athletics, and she also gets to compose and Perform the Ballad of the Farthest Rock Throw.
She also gets to invent, and then celebrate in song, the Brill Bend. How high does she jump on an Athletics check of 54?