@karter,
Whilst you're doing your initial screen close scrutiny, it would probably be best if you set the screen refresh rate to 1 (Settings - Reading settings - Refresh screen every 1 page). Then you can eliminate eink ghosting from the equation. Also after some actions (e.g. tap screen to get top & bottom menus then tap again to dismiss them), the screen isn't fully refreshed so the text 'under' the top/bottom menu areas may not be fully black. If you turn the page, the full 'page black flash' should even everything out again.
Another consideration is that some fonts have smoother outlines than others. Try a few different ones. Having font weight set to 100% seems a little excessive to me but each to his own
Having said all that, eink evenness and contrast is never going to match a hi-res phone/tablet screen so one has to set expectations accordingly. Eink doesn't suit everyone.