Having read all but one of the Typhon Pact novels, I have to say that I don't particularly think you can consider the Typhon Pact narrative as it has played itself out thus far to be an 'arc'; given that each of the novels which have been released under the Typhon Pact banner thus far has been focused on or skewed towards a particular, specific series (DS9 for RBoE, ZSG, PoN, and RtD; TNG for PoD and TSW; and Titan for StF) and either furthered existing storylines or established brand-new ones for the specific series around which they are focused, the Pact comes off less as a single, connected story arc and more of a 'campaign setting' (to use an RPG term) for the ST universe and Trek Lit as a whole.
That's an interesting analogy. I'm not sure it's an ideal analogy, though, since "campaign" implies something with a definite end goal. The Typhon Pact is just meant to be a part of the astropolitical landscape now, like the Klingon Empire was in TOS or the Cardassian Union was in DS9.
And I agree, calling the Pact-related books to date a single "arc" is imposing a retroactive reinterpretation on them. The first six installments in the Pact narrative -- A Singular Destiny
, the initial 4-book miniseries, and The Struggle Within
-- were all meant to be basically standalone books against a common backdrop. The 4-book miniseries was meant to be like the earlier miniseries where you could just read each book on its own and didn't need to read any of the others if you were only interested in one or two of the series represented. In TSW, I did try to mention a few threads from the previous books and sort of do a coda to the whole thing, but it was still largely a standalone tale.
But now, with the just-concluded duology, DRGIII has taken those formerly standalone books from the previous novel series and woven them together into a single narrative. PoN/RtD binds the Pact volumes (or at least RBoE, ZSG, and PoD) together more tightly than they were originally. So those five books constitute a central "spine" of continuity within the broader Typhon Pact "campaign setting," as you call it, that also includes ASD, StF, and TSW as more standalone works (and some peripheral mentions in Watching the Clock