SpeedScale seems to be the property to use, I don’t know why it doesn’t work for you. SetSpeedScale is the same as assigning SpeedScale (it’s just the internal setter used in the property).
You could also change Frames.SetAnimationSpeed("Anim Name", speed), but if you re-used the same SpriteFrames resource elsewhere (or duplicated the node) it will change it everywhere it’s used.
Note: setting SpeedScale will have no effect if the animation is not playing already. i.e doing SpeedScale = 1f will not start the animation, and 0f will not stop it.