It need not necessarily run EPOC16, though a battery-optimised OS would be quite useful.
I'm using a 13" e-ink tablet for most of my mobile use (reading, no social media, absolute minimal number of account-based tools), and it's quite good. A bit large for a pocketable / PDA device, and the fact that there's no keyboard which could integrate into a folding-but-usable case as I'd had for an earlier Android tablet[1] is disappointing. Apple likewise.[2]
I'm not looking for a phone (though with a headphone jack, the device might serve for vox comms), but a device on which I can read and create text and some images.
E-ink is battery-friendly, highly readable, works well for greyscale graphics (I often, though not always, forget that I'm looking at content which presumes colour), is highly readable under direct sunlight, and can be read in low-light conditions with a modest frontlight.
Purism and Pine may deliver here, though I'm not holding my breath.
Keyboard is essential to writing. Touch input sucks.
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Notes:
1. The keyboard itself was ultimately a disappointment, as was the tablet. The form-factor and ease of flipping from touch-based to keyboard-based use was quite appealing. More: <https://ello.co/dredmorbius/post/lqgtwy_rhsfbdh5cdxb1rq>
2. The two problems are iOS's lack of a Linux-like userland, and yes, I'm aware of ish, and the fact that despite some otherwise delicious keyboards, keys critical to Linux/Unix tools and conventions are missing, most notably <esc>. Android/Termux remains superior, despite my many reservations against Android.