I’ll be away from the forum, but you might want to check out the lack of the 12 nidānas outside SN 12/SA 3. Much like the aggregates, the 12 links basically have close to no strong basis outside the SN/SA. There has already been some surveying with Chinese parallels in the thread here.
I, like you (I think), get the feeling that the SN/SA is the Sangha’s more technical, oral manual for grouping advanced dhamma category teachings together and making these philosophical systems consistent and more inclusive of the thought-world expressed elsewhere in less concise formats. It’s very likely some of this systematization occurred later on, whether in the Buddha’s teaching career or slightly after. It would have to be very early on though. I do think there is also a clear intentional grouping of types of content though as well at play, and a gathering of discourses into one collection to keep them together.
Mettā