Hi @Dhammabodhi ! Just to play devils advocate to @sujato for a minute, there are in fact lots of reasons to think that the SN/SA collection is later, not earlier, than DN/DA and MN/MA. A few of those reasons;
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DN/DA makes no mention of 12DO, mentioning only 10DO (in DN14 and DN15) and even shorter lists (in DN1 for example). SN/SA acknowledges both 10DO and 12DO, so if we think that the shorter list is likely to be the earlier one, DN is likely earlier than SN.
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DN has very little evidence of 5A as a doctrine and the major place it is found, it seems to be inserted (DN14). The presence of this doctrine in some nikayas and not others might be interpreted as implying that the doctrine developed after the nikayas from which it is absent had already been composed.
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SN names suttas from DN and qoutes from them, (SN41.3 and SN22.4) but no DN suttas ever mentions an SN sutta, this could indicate, again, that SN developed after DN.
there are plenty more reasons to think that SN is later, the fact that it has an elaborate, thematic structure is itself suggestive of development and scholastic sophistication that seems antagonistic to what we would expect from the way communal chanted literature would develop in a community, and there are plenty more arguments too.
a couple of threads may be of interest;
Anyway, good luck with your exploration!!