Most of the visible jumps are of just 1 level (for example from gray (120,120,120) to gray (121,121,121), so they cannot be made softer. In absence of noise (which is the case) this means visible posterization, and it cannot be avoided.
This is a good example of the 'more levels' falacy many people insist when talking about ETTR.
ETTR means better SNR, i.e. less visible noise. If a plain area of a scene (e.g. your sky) is sufficiently clean of noise because of ETTR, once converted to an 8-bit output (e.g. JPEG) the bands will be visible like in this case. The RAW file had a lot of levels thanks to ETTR, but far from preventing posterization, the low noise achieved through ETTR produced banding on the 8-bit output.
Contrarily to what many times is said, posterization usually comes from insuficient levels on well exposed areas in 8-bit, never from insufficient levels in the RAW file since noise always dithers captured information, no matter the exposure achieved.
A possible solution is to add noise in the sky to make the bands invisible.
Regards