[Quoth]

There are other reading apps where you might not be able to read on a non-android ereader, but if I buy a book, I can read it on anything. If it's only borrowed, then likely I need an app unless the reader can be registered with ADE or has Overdrive.

Obviously I use Calibre and have Kindle, Kobo, Sony (and now a Y-Ben P47L!), but during lock down I thought I'd read library books on eink instead of the table so bought the Mars. The basic meebook P78 seems a relaunch of it. It doesn't support a digitizer/stylus. I'm told the P78pro does support it,

There are at least 5 kinds of incompatible pens/pencil/stylus digitiser systems. Kindle Scribe and reMarkable use Wacom EMR. Some tablets & screens use the same Wacom as the add-on Wacom USB & serial tablets for PCs. I have both types of Wacom and they are absolutely incompatible.
The Kobo Elipsa, Kobo Sage and MS Surface can use similar or identical pens that take a cheap (20c for 6 months) AAAA cell. But some MS pens also have BT for One-note and are rechargable. Also 3rd party pens for Surface are about 1/4 price or less.
Of course the Apple Pencil is different. A 3rd party one is about 1/6th price and just as good. Both are rechargeable versions only.

Both common types of Wacom pens are powered by the host digitiser and have the flaw that they tend to signal position even if you are not pressing.

If you want colour, then a really good LCD or OLED and turn the brightness to suit the ambient light makes colour eink look stupid. There are LCD & OLED matt surface screens without stupid shiny finish. Refections cause headaches and eyestrain as you unconsciously change focus, especially if the thing reflected or the screen is moved.

Outdoors the colour eink will beat LCD & OLED but not Quantum film based panels. The newer better resolution and colour range eink-like digital papers are really very slow.