I think its a bit early to say its not viable. Header tanks are there precisely to solve this. The sloshing in the header tanks should manageable in a tank size

I think its a bit early to say its not viable. Header tanks are there precisely to solve this. The sloshing in the header tanks should manageable in a tank sized correctly, specially if you are willing add some extra baffling to the header tank. We should realize that after the first failure SpaceX already had built a couple of prototypes that were 'wrong'. So they had to basically do hacks to figure out how to fly them at all. I would assume with the next generation they have really thought about how to do the header tanks and redesigned them and how the pressurization of those work. They are seemingly learning about some interesting things about liquid methane and bubble formation in liquid methane. The baffles they had apparently caused a vacuum bubble in SN8.