The iPhone doesn't play well outside Apple ecosystem to download ebooks to something, and not all ereaders support Google drive, which is an added complication.
Also even on Android phone (Android 4.x to Android 13), you need an ereader with Mass Storage (so not Kindle Scribe, Onyx Boox, Pocketbook android models or reMarkable) and an OTG adapater and a File Manager. I've managed it to a PaperWhite 3 (unlike Scribe it's Mass Storage) and a Kobo Aura H2O on Android 4.1 Sony phone with a Micro-USB OTG USB-A adapter and a USB cable. With Android 13 I had to add a USB hub with also a USB memory stick to get the Kobo Sage recognised. Then I was able to copy on a downloaded epub and also copy off the conf file and open it in Jota (free Android text editor).
I can't see that Google Drive helps and only having an iPhone is limiting unless you only read on iPhone apps.
Hence either a 7″ approx Kindle or Kobo, or if 10″ plus really needed the TCL Nxtpaper 11 tablet, (under €240 in shipping in EU) and is almost as good as mono eink and does everything except deDRM and run Calibre! Far better than a x5 price iPad (we have compared side by side).
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