While the lack of Regeneration on the official version removed one of the Tarrasque's USP's having to use a Wish to kill it (nice implementation of that A5E ); the omission that always irks me the most is the removal of its Sharpness Bite - maybe that's too dangerous for today's gamers. But I always saw that as its signature attack.
Having played a lot of Epic Tier (and witnessed a Barbarian defeat a 2014 Tarrasque one-on-one) In terms of damage, for a CR 30 monster I estimate around 310 should be the default damage output:
For a CR 30 version, off the top of my head I'd probably have:
Multiattack: Bite + 2 Claws/Stomps or Spit Boulders
Bite: 210 (20d10+10 maximised for Sharpness) + Sharpness effect on a 20 + Grappled
2 Claws/Stomps: 80 (20d6+10), Claws knock a target prone, Stomps deal 130 maximum damage vs prone targets.
Spit Digestive Boulders (begins with 3 boulders, Recharge one Boulder 5/6): Range 1 mile, Each Boulder deals 35 (10d6) Bludgeoning + 35 (10d6) energy (either acid, fire or poison) and the blast covers a 40 ft. radius. Can spit 1-3 Boulders per round.
Combat Reactions/Legendary Actions (Choose or Roll 1d4, 1d4+2 when Bloodied)
1. Cause Tremors: Targets on the ground within 30 feet take 35 (10d4+10) bludgeoning damage and must make a STR save or knocked prone.
2. Tail Sweep: All targets within a 60 feet cone take 45 (10d6+10) and are Pushed a number of feet equal to the damage and knocked prone.
3. Your Next! The Tarrasque fixes its gaze on one target as if to say "Your Next!" The target must make a WIS save or become Frightened. Failing the save by 5 or more Disheartens the target (Disadvantage on saves).
4. Tail Slam: 1 target within 60 feet 65 (10d10+10) bludgeoning + knocked prone + must make a CON save or Stunned til end of next turn
5. Terrifying Roar: Those within sight of the Tarrasque must make a WIS save or be Frightened. Those failing the save by 5 or more have a Heart Attack and take Necrotic damage equal to their Bloodied value.
6. Rush! The Tarrasque moves its Speed in a straight line towards one target (making a Stomp attack against all those in its path of destruction) and it makes a Bite attack against its target.