I should have added this when I first posted, but it went over my head-
You tried a Shimano rd-m6000.

Looking up the spec's for that, it's=
1. Only intended for an 11-42T cassette. 31T diff.
2. A 10T front difference. Total wrap & capacity is spec'd at 41T.
Basically, one you've down shifted off the 2-3 highest (smallest cogs), the RDER is expecting to "see" larger cogs.
It's designed to move at an angle appropriate for an "oversize" cassette, relative to standards of a few years ago when 34 was your max on about anything.
https://bike.shimano.com/en-NA/produ...-M6000-GS.html

My Ebike has the M-592 for its (9 speed) 11-36 1X.
https://www.google.com/url?sa=t&sour...cftaXggpkDsE4F
I currently have it with a cobbled together 13-30 cassette and made no adjustments other then a couple clicks on the adjuster.
It works adequately for me that I'm not going to try to "improve it". I do have a new 12-26 to install when the mood strikes.