Everyone's eyes are different. I suspect mine are pretty sensitive (I even had eye strain with a lighted e-ink reader). Hopefully you can find what works for you.
Try playing around with different custom colours in your android app. And yes, turn down the brightness even further than 20% you tried before.
Suggestions:
Beige/khaki/tan/off white background with brown, navy, or charcoal font (or even mid/lighter tones of these).
Navy or darker blue background with mid or light grey;
Brown background with beige/tan (I know you said you don't like reverse black/white, but basically try the same idea of 'night mode,' but with less stark contrast).
If you read mostly or all fiction, you could give e-ink a try if you can find something in your budget and at a retailer with a good return policy (Kindle, Nook, Kobo or discontinued Sony PRS series).