Oooo.
Wow, this sounds awesome.
I hadn't thought of the concept of writing into a multiple-column layout before. That's really really neat. It keeps the individual runs of text short (faster for proofreading, I've heard), scrolls less, and lets you keep more context on the screen.
I'm definitely very interested in this when it comes out, I don't currently have a tablet-size device but that probably won't be the case for too long and I can see myself giving this a go. A single-column option for phones (in landscape mode) might be practical.
I do completely agree that Android feels structurally broken (for want of a better way to put it) in terms of application support - it honestly feels like someone figured out how to bolt WebKit onto a feature phone and add capacitive touch and eye candy, while (somehow???) maintaining the same "smol device" fundamentals of "toy" and "not full PC". IMHO the PC experience isn't really one single cohesive vision, but rather the emergent result of years of many thousands of little independently-evolved self-sustaining threads and ideas (particularly in UX design) intersecting and bouncing off each other in a sort of cohesive balance that resulted in (comparative) miracles like Windows XP. Microsoft et al didn't invent the fundamentals of the ideology and language that emerged; rather, they just figured out how to capitalize on the net result and realized that if they knee-capped it too heavily they'd kill the thing they were trying to benefit off of.
From a distance it kinda looks like Apple/Google are trying to (re)invent everything - conventions, expectations, design language, (tens of?) thousands of tiny details, etc - from first principles, without giving sufficient consideration to whether their fussing about accidentally edits and rewrites the technological aspects of the goal narrative. I think advertising and the exponential over-valuation of user data are to blame for the current insanity - and sadly so, since it's obvious that won't change soon. I think the fundamental incentive to refine and optimize for technical experience/competence has become diluted by All The Ad Revenue™ from the current status quo, producing the current ecosystem, app store offerings, app quality level, fragmentation, etc. Basically everything seems to be working as intended, or in other words I don't mean that "current status quo is fundamentally bad and evil" (for want of a better way to put it), I rather mean that between ad revenue, internal politicking, Google/Apple/Microsoft competition, antitrust regulations, overall public perception of how invasive ads/tracking is, etc etc, the system as a whole has basically reached a steady state that simply doesn't have room for pure technical excellence, which I don't think will change until the ad bubble collapses. I'm tentatively hopeful that might actually happen in my lifetime, although as a counterpoint, hopefully it doesn't take so long the world ends up in a WALL•E-style dystopia with everything paperclip-maximized around ads instead of garbage. Haha.
However, this all provides a unique opportunity for anyone prepared to make the effort to make apps that are actually interesting: can I interest you in a "Buy" button? :P
You mentioned cleaning up the source code - and while I certainly won't dissuade you from open-sourcing something like this :D (it would be a genuine net improvement, and you could list on F-Droid as well), I'm reminded of the "source code is on GitHub, but ready-to-go APK in Play Store is $.$$" model, which I think could work quite well for this sort of thing. There are a lot of small, focused apps out there that have loyal followings.
Maybe you could do an "early beta" sort of thing to cue users to understand the first versions are free but you plan to change that at some point. (The Play Store management dashboards probably make it straightforward to grandfather everyone who started with the beta into free updates for life or something like that.)