I think one thing you’re missing here is that under such a system the defaults would likely become your locally hosted /c/books rather than the largest one. Even still you’d probably see posts from the largest books communities because /c/books@your_instance follows multiple /c/books@big_instance. Community blocking would likely still work as it currently does so any books communities that you were not fond of could still be blocked.
There is still the issue of where do you post and I think the answer looks something like:
Post in /c/books@your_instance if you want to talk to your neighbors
Post in /c/books@big_instances if you want to talk to everybody
Which is more or less how most people would decide where to post book stuff anyway.
Turning the fediverse button into an “open on my instance” with similar functionality to subscribing may also be a solution here. Bonus points if it’ll also open a comment on mastodon.
While I agree with the content of that article I don’t know if we should give up on Eugen just yet. The Mastodon team has not disclosed what their plan is regarding the groups rework currently on the mastodon roadmap. There is an old proposal here, but I think we have good reason to believe that implementation will be revisited. To that end, it is very important to advocate for the adoption of FEP-1b12 which is the standard that Lemmy uses.
It may also be a good idea to advocate for the adoption of FEP-d36d both here and on lemmy. This is a standard for group-to-group following. Effectively allowing communities to subscribe to other communities.
I feel like your best bet is ESPHome and the ESP family of devices. Last time I checked you could get the parts for a project like this off Amazon (with spare parts) for under 20$.