Each Artemis launch costs something like $4b (that's the incremental cost of a new rocket, it's much higher if you amortize the design costs).

IMO the program is not optimized for cost or sustainability, it's optimized for creating jobs in various congressional districts. Of course that provides a certain amount of political sustainability to the so-called Senate Launch System.

I just don't see a future where NASA can afford multiple SLS launches per year to maintain a continuous Lunar presence

I think that is the point, but whether this mission will actually do that is rather unconvincing.

After (and if) Artemis III lands on the moon and brings home the astronauts there seems to be very little planned on how we actually get to the moon base which NASA is claiming this will lead to, let alone the manned Mars mission that is also supposed to follow.

In other words, I think NASA is greatly exaggerating, and possibly lying, about the utility of this mission.