I want a reader mode that renders the page as if it were in an extremely tall window (ie. 10+ screens tall), then gives me a scrollable view of that static image of the rendered page. My browser should lie to the page on my behalf, and make it behave as if everything were already on-screen.

Materialistic[0] effectively does this (minus the screenshot part), not intentional I think (I belive it makes the webview as tall as the requested page and then uses the OS native scroll widget to add a scrollbar for it). The problem I regularly encounter with this is sites that have a vertically centered popup (cookie banner, newsletter, etc.), with a backdrop that obscures the whole page. You first have to scroll down quite a bit (half the size of the article) to be able to click the popup away.

[0]:https://github.com/hidroh/materialistic