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IMO a 21 Ethos would be a waste of field stop...the reducer you're using makes the light cone steeper (making it faster!) so at the same time the useful field stop of the eyepiece shrinks too. The wider you try to go the more you'll see issues.
I've read people say the 17mm Ethos with the reducer is a good combination, but never used it myself. At native f/10 it works well, but there are much less expensive options to achieve more or less the same thing.
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