I use https://miniflux.app and use that to sync NetNewsWire across my devices and across RSS readers. I'm using Reeder on my iPad, Miniflux on the web and sometimes NetNewsWire on my Mac.
I used Reeder for a year, but switched to Miniflux because I wanted an RSS reader that could be used outside of my Apple devices. I do miss having a mobile app of my reader, since Miniflux can sometimes be hard to navigate on a mobile device. I never seriously considered using multiple readers until now. Thanks for the accidental recommendation.