it sounds like the cassette doesn't have a good ramp setup. If the der can be set to over extend it's shift & pull itself back to alignment of the cog, that might get it to follow thru with all the upshifting. Downshifting will be hard to solve.
Prestacycle Uniblock Cassettes has been on my radar to try, but I want what the 11-32T offers but with a 34T at the end.
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-Oh Hey!