How about the halfway house of RTF, then? That way, you don’t get the problem of images messing things up, but you get the advantage of formatted text.
If there’s scope for you to do file manipulation in the Finder, you can use Terminal to do manipulations of this type:
`textutil -convert rtf -font Avenir -fontsize 16 *.txt`
(Convert all txt files in the current directory to rtf, changing the font to Avenir 16)
You could use the New Terminal at Folder Service to navigate to the relevant directory if necessary.
Another thing to consider: In DevonThink, you can select a chunk of text in a document and drag that out to a folder of your choice. (You might want to keep it in the same folder as the original document.) You could do that for multiple snippets in the same document, and then convert the snippets into the format you want for Tinderbox, going through the Finder if necessary. The snippets don’t need to be explicitly linked back to the main document, because DT’s search and See Also will notice that the text in the snippets is identical to part of the main document.
The unfortunate bottom line is that, whatever wonderful AI features DT has, at some point, with only limited time at your disposal, you need to read through something that is too big and long to be read easily. Making snippets for parts of the document that you have decided are important is a good first step at asserting some agency over the process, whether or not you take them over to Tinderbox. And, since your decision regarding which bits are important is not irrevocable—you can always go back and get another snippet from a section that you have subsequently decided is key—you can start moving snippets over to Tinderbox before you’ve finished reading the document.
I’m not sure this is helpful, but this is how I think of the DT – Tinderbox relationship currently.