MNT Pocket Reform
mntre.com
mntre.com
Others have implemented these via custom enclosures either 3D-printed or acrylic laser-cut, but the time and resources I'd need to invest for that are probably equal to or more than what I'd spend on just an MNT Pocket Reform.
Alternatively, I could get a tablet PC and just pair it with an ortholinear keyboard. This leaves the option open for x86_64, which is unfortunately a requirement for Proton Mail's bridge application. Ultimately I think a specialized device would be a more satisfying solution, even if it means having to use Proton Mail's web interface...
Though I'd personally require a smaller screen bezel in such a small form factor, that's too much wasted precious area.