So, I wrote one.
It shows one post at a time, makes the text bigger, and hides almost everything except the content of the post. There's a drop-down for navigating between the different posts and buttons for switching back and forth between reader view and the normal one. Here are a couple of screenshots.
Screenshots

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I wrote it mostly for me, so it's far from perfect and I can't guarantee that there aren't any bugs. Still, I thought I'd put it out there in case anyone else might find it useful. (It makes a huge difference for me.) Oh and things like the font size and colors are all hardcoded. If anybody else ends up using it, I may make things like that configurable.
I didn't publish it to the Chrome Web Store for various reasons, so installing it is a bit of a chore. If you're brave enough to give it a shot, here's how.
Install Instructions
- Download the file and unzip it somewhere that it won't be in your way.
- Open your browser and go to: chrome://extensions
- Click the toggle at the top right to enable Developer mode.
- Click the Load unpacked button that appears in the top left.
- Navigate to the phpbb-reader folder you just unziped and select it. It will appear in your extension list.
- Click the Details button on the phbpp-reader extension.
- Click the toggles to enable Pin to toolbar and Allow in Incognito/Allow in Private (if you use private/incognito windows .)
- To use it, open a topic on RA and click the icon on your toolbar that looks like this:
It should work on any chromium-based browser. It works on any phpBB site too, it just might not look great.
Let me know if you give it a try.