And, in case anyone is interested, a quick check against my most common use cases for the Kindle, Kobo and PocketBook apps. The core of my requirements is being able to carry a couple of dozen books around with me when I'm away from home while leaving my back intact and enough room in my suitcase for fripperies like spare clothes.
Can I use it to read PDFs? Kindle - usually yes if I email them to myself but it has a size limit and it's funny about some with DRM; Kobo - not as far as I can tell; PocketBook - autoloaded all PDFs on the device once I gave it file access. Kindle and PocketBook remember which page I was on, which is more than some PDF readers.
Does it have books I'm currently looking for? Kindle - all of them, Kobo - 2 out of 3, PocketBook - none of them, although in theory I could buy two on Google Play and load them into it. (Those being Forgotten Ruin by Jason Anspach and Nick Cole, Do Less by Rachel Jonat, and Sette Breve Lezioni di Fisica by Carlo Rovelli; PocketBook did offer me that last in German or Russian, but not Italian - surprising for a Swiss company.)
Can I buy books from within the app? Kindle - no; Kobo - not at the moment but I have managed it once before; PocketBook - allegedly, but it didn't have anything I wanted to buy.
PocketBook does get points for complying with GDPR but appears to ship my data to Google and Microsoft, making compliance meaningless. The English translations of its T&C are bad enough that I thought at first it was a fake app put out by scammers. At the moment, however, I'd say it's a decent PDF reader but not much of an e-reader.
It appears that wanting to read multiple file formats and buy - sorry, rent - the books I want from within the reader app is asking too much.