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How about the freehub body, pawls might be gummed up?
This would be my bet, unless the chain or the cassette is really gone. Chain too long only would be bad in the small cog, bad chain line it's only appreciable in one or the other extreme. Wearing out all cogs of the cassette it's quite an achievement too, you only get a few gears slipping at the beginning.
It could also be that the derailleur is not mounted in the right stop in the hanger.
yeah i didn't get the 1x as a reason, Phildas worded it better than my brain could. How about the freehub body, pawls might be gummed up? Mine are currently and I'm getting the occasional slip.