Here's a ss of Lapras' base stats for measure https://imgur.com/a/JDdZtYR

and it's most common EV spreads https://imgur.com/a/LWu5eQt

Assuming this is for doubles, there's no need for Dragon Dance since the pace is too quick to allow much setup. Also Lapras doesn't benefit much from an attack boost or a speed boost unless you go all out max ATK/max Spd. Invest those attack EVs in to HP, Defense or Special Defense

If you can Gigantamax it, it's crucial to have an Ice move. G-Max Resonance is too strong of a move to not have since it basically sets up a Light Screen and Reflect (-50% all move damage received) that cannot be broken. Freeze Dry, Ice Beam and Blizzard are excellent choices. Light Clay for this reason might be a better held item over Mystic Water

With Mystic Water, Sparkling Aria will do a lot of damage but keep in mind it also targets your own teammate. You can play around this or just switch to Hydro Pump

Recommend: Your choice of Nature & EV spread from above, Light Clay item

Hydro Pump, Freeze Dry, Thunder, Life Dew/Helping Hand/Protect as final move for a very powerful Dynamax option