New chain skips when old chain was replaced at 0.75% mark
To me, that looks pretty worn.  My guess is that as the chain elongates, it grinds further into the gear valleys, making the &quot;flattened&quot; profile.  The chain and cassette become mechanically paired, wearing together until the chain breaks.  The cassette longs for its lost chain and will not take another mate... er, is mechanically incompatible with other chains.   The wider profile probably leads to periodic instances of 2 rollers in the same gear valley, sort of riding the top of the valley and unable to lower a single roller.
So, options...  1)  Clean frequently, and maybe consider a different lube.  Lube minimally, just on the rollers.  I use Muc-Off dry ceramic for our gritty trails, but it's very dry here.  I have no idea how it'll handle your wet grit.  2)  Replace the chain sooner.
For comparison: