I have just purchased a used e-ink tablet and I can recommend using a BOOX. As you can download the Obsidian APK and use handwriting through the BOOX keyboard instead of having to export a .txt file into a .md file and into your vault. As handwriting is built into the keyboard, it will work in all apps and filetypes. It is still in the keyboard, so you only have limited space instead of the whole page, so I recommend it for larger e-ink tablets (10 inch and up).
I use the emerson journalling method, which I always did on paper, but now I can make past weeksheets searchable.
Here is a link to the method: how i journal — Hello Emerson