My own sense is that "You should X" is primarily a call to action roughly synonymous with "X!", and not really asserting a proposition at all

I see your point, I think. However, that's not how the OP treats it:

my current thinking is that "should" usually means "better according to some shared, motivating standard or procedure of evaluation", but occasionally it can also be used to instill such a standard or procedure of evaluation in someone (such as a child) who is open to being instilled by the speaker/writer.

which is either/both asserting a proposition or/and taking action (of convincing someone to do something).

There is a big distinction between a statement about possible worlds ("is"), "should " and "do!", of course. In the OP's link it's discussed as normativity vs norm-relativity. Unfortunately, it has the standard philosophical shortcomings I alluded to in my other comment: it does not attempt to formalize it and instead goes into various historical descriptions and the differences of opinions between several equally confused schools, trying to swallow the whole thing instead of taking a careful manageable bite. Predictably, as a result, the whole thing gets retched back up undigested. Which is justified by calling it a "survey". Well, I am probably too harsh.