@houz, sounds like you’ve already made the prudent decisions.
Thumbnails are a PITA, both display and embedded in the image file. In rawproc I’ve chosen not to do either, but I just have one customer… 
Actually, I’m investigating embedding thumbnails in output images, but if it starts to get hard, it’ll be pushed off for other things, like lensfun (a library for which I’m finding that, for developers anyway, really shouldn’t have the word “fun” in its name… )