FUCK! Finally beat Sans and "played" to the end. Must have taken like 6 hours and attempts in the triple digits. This is genuinely the hardest boss I've ever fought. I guess there would be technically harder bosses that have made me quit entirely, like Hades on Chaos difficulty in God of War 3, or Gwyn in Dark Souls on a soul level 1 playthrough. But of those the former feels more cheap than anything, since it's essentially a one-shot fight for the player, and on the SL1 playthrough I was deliberately gimping myself, and the only viable tactic was parrying. But Sans felt entirely dependent on my skill, and as such I never went "fuck this", but continued again and again, not even turning the soundtrack off. I must have spent like 5 hours of my life listening to Megalovania on loop in that fight, and yet it never got boring.
Sadly, I'd gotten spoiled about the ending beforehand, so it wasn't nearly as effective as it should have been. Shouldn't have watched all those LPs of it... though I thought Chara appearing at the end was an easter egg instead of a central part of the ending.
And I picked "do not" at the end. It really is a fitting end to the genocide run, since even Sans says there's nothing for the player at the end. And it's literally nothing.
What a fantastic game.