Typically, for damage against general targets, you’re looking at:
Artoria and Mordred have interludes, so np1 Artoria = np2 Arthur, basically.
Artoria has Charisma, so she’ll outdamage Mordred by just a hair. Typically it doesn’t matter and Mordred’s kit is so much better (Secret of Pedigree is such a good skill), so if you’re going to focus on one, I’d focus on Mordred. Recommended skill level is 1 --> 3 --> 2, but max 3 before for you max 1.
Arthur will outdamage Nero because of Mana Burst and his higher stats. Between Arthur and Artoria, the NP damage difference really is impotant, but Arthur has a better farming kit due to his battery and higher base stats.
Since your Nero is np5, she’ll do comparable damage to Arthur…but Arthur will definitely outdamage her with his Mana Burst because of his higher base stats.
Re: Arts vs. Buster, the damage is the same. An AoE buster NP has a 300% multiplier…but Buster has a 1.5x multiplier, so that’s really a 450% multiplier effective. Arts has a 1.0x multiplier, so that’s still a 450% effective at np1 without an interlude. Nero at np5 will deal 900% damage on her NP, which is the equivalent of a pre-interlude ST NP. That’s a good bit above the 600% effective your SSRs are working with, but again, their in-kit steroids really do make a difference.
(This is all assuming Imperial Privilege doesn’t proc.)
If Imperial Privilege does proc, I’d say she’s about a strong as an np1 Mordred.
For future reference, you can eyeball these kinds of things with the Strongest NP charts, which is what I did: https://grandorder.gamepress.gg/strongest-np-against-lancers