Right now, Kobo is the best, IMHO. Pocketbook is overpriced and underpowered, Onyx Boox run locked-down Android, Amazon is full of customer-hostile practices and spyware.
You can't sideload on the Onyx? I've been comparing Kobo to Onyx and I saw that Onyx does support the Google Play Store, which is good enough for me. I'm guessing what you're alluding to is no sideloading of apks.
You can sideload, but the interfaces are weird and it's not anywhere near as open as a normal stock Android, let alone LineageOS or a real Linux like Kobo/Pocketbook (my older Kindle Paperwhite also runs some flavor of *nix IIRC, but not as easily hackable, and no idea about newer versions).