I agree that funding summaries is worth doing and that any important content should exist in the forms you listed to promote discovery and awareness—audio version, EA Forum linkpost, Twitter thread, Tiktok video, etc.
Besides the Handbook, the CEA has this introductory page which strikes me as absolutely fantastic. Maybe they ought to link to the introductory page from the beginning of the handbook?
Point of possible disagreement: I think that the deficit of good summaries looks less severe than you've suggested if one's main heuristic is to look at Google search results rather than EA Forum results. When I imagine someone time-constrained trying to find out what Doing Good Better says, for example, I imagine them looking for the book on Google, and to me the results seem not too bad. (A few clicks took me to the CEA's introduction to effective altruism, in fact!) If I wanted to find out more, I could go read the book.
About the issue of you not being sure if you want to click on a Forum post, that sounds like a matter of UI on this site specifically. Are you imagining some kind of descriptive text that people could see? On computers, there is hover text of the first bit of the post. If people summarize their own posts, maybe that partly addresses the issue?
Edit: You mentioned The Precipice not having a very good summary on Wikipedia. Would editing Wikipedia articles on major EA works (or even topics connected to EA) be another action someone interested in this could take? Is there something like a Wikipedia editorial policy that would prevent us from doing this?
I agree that a CC-BY licence is better (without the "noncommercial" restriction). I am chipping in because I'm not biased the same way as you are, except that I read content on the Forum much more often than I write it :)
The purpose of copyright was to encourage people to make creative works. We don't rely on financial interest to encourage people to post here—it's a prestige-based system instead, which seems to be working very well. Moreover, if we want the Forum to be a vehicle for disseminating ideas, copyright rules militate against that. I see many more reasons for switching to a CC licence than not doing so.
As for why I prefer not having the "noncommercial-only" restriction, it's because I don't want people not to reuse work from the Forum (with attribution) because they're unsure whether want to do with it counts as commercial. We should err on the side of sharing info freely.
Update: I do, however, agree with those who think that the CC licence should apply only to new threads. It would not respect the consent of previous contributors to put a CC licence on content already on the Forum, and I'm not sure it's even legal to do so.