I find it's not just a question of genre - some authors are simply easier for me to read. I'm just more comfortable with certain writerly voices.

If you like nonfiction, you could try collections of essays or books with clearly delineated sections so that you can tackle one part at a time. It also helps to have a few options on the go, so that you can read more challenging prose when you're feeling better, and easier/lighter/shorter writing when your energy level is lower (such as at night).

You can also use skimming strategies where appropriate or necessary, such as reading only the first (topic) sentence of each paragraph to get the gist of the material.

Weirdly, I find reading in my second language enjoyable, in part because I want to keep my fluency up, although I have to be careful not to overdo it.

I too am hopeful that my reading stamina will continue to improve as I recover. In the short term, I have simply adjusted my expectations - for now, I think reading even a very modest amount each day is a good thing (just as a short amount of walking is a good thing), so long as symptoms aren't being triggered.