Lugia's most useful niche is in soloing Machamps, and yours has a good moveset for it, so keep it as is.
Sky Attack is preferred as it's the higher damaging moveset with faster simulated clear times, and it has less energy waste because it's a 2-bar move which also makes damage output smoother - if the timer is low being able to use Sky Attack over Future Sight might mean the difference between winning or losing, but Future Sight isn't far behind. If you're low on charge TMs it's fine to keep it with Future Sight.
Future Sight can help if you're using it for maybe Gengar, though Gengar is an easy level 3 that there's lots of substitutes for Lugia - i.e. if you already have a Lugia with Future Sight and powered then it's fine to bring to solo Gengar with, but don't go out of your way to get one for that purpose.
There's other movesets that are situationally useful against other raid bosses but Lugia's mediocre damage coupled with the fact that damage bonus is retained upon rejoining makes tanky Pokemon unnecessary.
