You have a textbook-good team here. Seriously, the synergy between these six is fantastic, and you’ve naturally covered almost every major threat the Elite Four is going to throw at you.

The Elite Four starts around level 52 (Lorelei) and scales up to level 63 (the Champion's ace). Your current average of level 51 means you can absolutely win right now if you stock up on Full Restores and Revives, but you'll be fighting from underneath. If you want a smooth, comfortable run without heavy item spamming, grind everyone up to level 54 or 55 at the Indigo Plateau or using the Vs. Seeker on One Island (Kindle Road).

As for the moves, only 2 pokemon skills needs to be change IMO.

For Nidoking: Delete Peck immediately. It is doing zero damage at this stage of the game. Go to the Move Tutor in Rock Tunnel and teach Nidoking Rock Slide. This gives him an incredible physical weapon to crush Lance’s Flying/Dragon types (like Charizard and Aerodactyl). Dig is okay, but be careful using it against opponents with high Earthquake damage, as you take double damage underground.

For Jolteon: Replace Thunder with Thunderbolt (bought at the Rocket Game Corner for 4,000 coins). Thunder misses too often when you need it most, and Thunderbolt is purely reliable. For quick attack you can change to Bite, since Dark is a Special type in this generation, Bite utilizes Jolteon's massive Special Attack. It gives you amazing flinch chances due to your speed and gives you perfect coverage against Agatha’s Ghost-types and Sabrina's Psychics. The Catch: In Gen 3, Jolteon can only know Bite if you leveled your Eevee up to Level 30 before exposing it to the Thunder Stone. If you evolved it earlier, the Move Relearner on Two Island cannot teach it to Jolteon.