I think rationality absolutely must confront the question of purpose, and head-on. How else are we to confront it? Shouldn't we try to pin down and either discard or accept some version of "purpose," as a sort of first instrumental rationality?

Why do you think it needs to be confronted? I know there are many things that I want (though some of them may be mutually exclusive when closely examined) and that there are many similarities between the things that I want and the things that other humans want. Sometimes we can cooperate and both benefit, in other cases our wants conflict. Most problems in the world seem to arise from conflicting goals, either internally or between different people. I'm primarily interested in rationality as a route to better meeting my own goals and to finding better resolutions to conflicts. I have no desire to change my goals except to the extent that they are mutually exclusive and there is a clear path to a more self consistent set of goals.

There's little to discuss if you don't, because "everything is permitted."

To the extent that we share a common evolutionary history our goals as humans overlap to a sufficient extent that cooperation is beneficial more often than not. Even where goals conflict, there is mutual benefit to agreeing rules for conflict resolution such that not everything is permitted. It is in our collective interest not to permit murder, not because murder is 'wrong' in some abstract sense but simply because most of us can usually agree that we prefer to live in a society where murder is forbidden, even at the cost of giving up the 'freedom' to murder at will. That equilibrium can break down and I'm interested in ways to robustly maintain the 'good' equilibrium rather than the 'bad' equilibrium that has existed at certain times and in certain places in history. I don't however feel the need to 'prove' that my underlying preference for preserving the lives of myself and my family and friends (and to a lesser extent humans in general) is a fundamental principle - I simply take it as a given.