Is it the case that these devices are converging on 16:9? I don't know about the supply chain, but there seems to be no lack of e-ink tablets at A5/A6 sizes and/or with better ratios than 16:9.

Remarkable has the roughly A6-sized Paper Pro, Kobo has three e-ink devices with styli and good screen ratios, and Supernote has models named A5 (and A5 x2) and A6 after the paper sizes. I think the options are quite good.

Paper Pro is roughly A4-sized