Owning a mix of Ethos, Nagler and Panoptic EPs myself (including the 35mm Pan and 20mm Nag) I'm not sure I'd spend the small fortune an Ethos cost when you already have the 20mm Nag... It's tiny and great to look through. The Ethos will be massive and significantly change the balance on your DOB.
We have a 21mm Ethos and a 20mm Nag - I far prefer using the 20mm Nag on our dob, we tend to use the Ethos in the refractors.
Perhaps consider a 13mm Nagler to fill our your set nicely ?
Of course I have no idea what other astro extras you have already, but if you don't have a good adjustable chair, a laser for quick collimation (still expect to tweak by eye) and a good dew system (you're going to need this in the flinders) - spend some money on those first. Next buy the three volume 'Night Sky Observers Guide'. Also if you don't have a freezer suit or similar very warm observing gear, that can make a huge difference to enjoyment and how long you stay out looking.
Oh final thought, adding an Argo Navis to my DOB was perhaps the single best thing I've ever spend astro money on.
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