I think it’s important that those upfront costs aren’t his First Dark Side Experience™. It’s still the Tuskens.
What I’m positing is that the guilt and self-hatred of that has permanence, informs his internality, even by the time of ROTS. When he bows to Palpatine, he can agree to the terms because he’s already spent the past three years haunted by what he’s done, already knowing well the shape of evil inside him. It’s what he’s been running away from, to reputation as a hero (a good man), and what he hopes is masterhood (vindication as a Jedi).
I get what you’re saying. I think the problem is how this is portrayed. Ultimately, the script doesn’t present Anakin’s journey to the Dark Side as a linear path that goes from murdering Tuskens in AoTC to murdering everyone in ROTS. The Tusken massacre almost seems irrelevant because there’s no visible consequences to it immediately after the scene where Anakin confesses it to Padme. Anakin immediately reverts to standard good guy protagonist during the Geonosis arena scene. Then the next time we see him at the beginning of RoTS, he’s even more of a standard good guy protagonist, doing standard good guy protagonist heroics. There’s no linear progression. Then he has a moral problem with beheading a defenseless Count Dooku, even though he already mercilessly killed a whole tribe 3 years ago. The movie acts like Anakin’s morals are really being challenged when Palpatine asks him to kill Dooku, as if Anakin wasn’t already a completely psychotic killer.
So Problem 1 is that the films themselves don’t show any linear progression after the Tusken massacre - they almost act like the Tusken murder never happened. (I think Palpatine brings it up one time to Anakin in Revenge of the Sith - that’s the only time after AoTC the Tusken massacre is even acknowledged.)
Problem 2 is that the Tuskens have always been presented to the audience as mindless inhuman monsters. This further muddies the waters, because presumably we have reason to believe the Tuskens are sentient beings and murdering this whole tribe should be considered a genocidal atrocity. Anakin murdering the Tuskens should push his character way beyond the moral Event Horizon. But the film doesn’t really seem to treat it that way. The film treats it as a “BAD thing”, but not a “REALLY BAD thing”. After Anakin confesses, Padme is just like “well you know these things happen.” Then the issue is dropped, and Anakin reverts back to standard good guy protagonist. Furthermore, Padme, while at times naïve, is presented in these films as a moral compass, advocate for democracy, and voice of reason, especially in RoTS, so if she has no problem with Anakin mass-murdering a whole village, as an audience member I come away with the sense that the Tusken massacre wasn’t even supposed to be that bad.
So when Anakin finally goes “full Sith” mode in RoTS and agrees to murder everyone, it’s difficult for me, as an audience member, to factor in the previous Tusken massacre as a stepping stone towards this decision. Because as an audience member, by the time I get to the Sith conversion scene, (A) I already watched almost half a movie that was acting like the Tusken massacre never even happened, and (B) the previous movie kind of acted like the Tusken massacre wasn’t such a big deal anyway.