Glad the E-Ink option sorted out the contrast. I read on a Boox Go 7 with that enabled myself.
I’ve managed to reproduce the full-screen problem, and the overlapping button turns out to be Einkbro’s own. In full-screen mode it draws a small round floating button in the bottom right corner of the screen, on top of the web page, and that lands right on Readlang’s next-page arrow. It also swallows the tap, so tapping there drops you out of full screen instead of turning the page.
You don’t need that arrow though. Tapping anywhere down the right-hand edge of the page turns forward and the left-hand edge goes back, so tapping higher up the edge keeps you clear of Einkbro’s button.