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to be honest, I use the calibre ebook viewer without bothering with calibre itself (so far) and I find it to be good at what it does

I'm also using Calibre, but the ebook viewer is really clunky, I'm not a big fan.

You might want to check out Kavita, the epub reader is quite good.

The ux for the settings and such are ...special, but it does what I need it to do and I haven't sought out other options