As many of the missions that were drivers for the formation of this group either end or reach a state where significant new discoveries start to (arguably) decline, my thoughts move to finding summaries of these missions that step back from the day-to-day and look at the bigger picture.

Since I am not tightly connected to the space community, I am not sure how to find this information. I am wondering if there is any interest in creating some space here to collect key summary documents in one place? I am particularly interested in freely available documents that are targeted at scientifically-oriented people, but not those who are insiders or experts in the field.

For example, is there a summary of all we have learned about the Saturn's rings or Titan's lakes?

I think you’re describing Wikipedia. The reference section of a Wiki page is a superb resource.

I.e.

https://en.m.wikipedia.org/wiki/Rings_of_Saturn

https://en.m.wikipedia.org/wiki/Titan_(moon)#Lakes

Yes, it's been a great resource for intensely interested laymen like me (I even edited some pages back in the day!).

Many missions still have their home pages up and running, of course. I like this nice summary from the MESSENGER team of their discoveries, for example: http://messenger.jhuapl.edu/index.html#top10
Operating missions also have a wealth of resources too, usually in a 'Science' subsection.

Thanks. The only thing wikipedia doesn't do is tell me when it gets updated.

Yes, it does

https://en.m.wikipedia.org/wiki/Help:Watchlist

Well, I'm writing a book about Titan's seas.... and I think there is a book on Saturn's Rings in the works