I found a very comfortable position, but it depends on a reading app that supports continuous scroll (fortunately, both the Kindle app and the Apple Books app have this setting).

I lie down with my head slightly propped up on a pillow and the tablet on my lower chest (or upper abdomen) and keep my eyes focused on the top of the reading view area. Instead of moving my eyes or my head to progress through the book, I scroll up in chunks of 2 or 4 lines, as needed.

I have to hold the tablet steady and also scroll the content, but somehow there's no strain to my arms or hands in that position.

I am a slow reader, but I can make tons of enjoyable progress through a book using this method.