I think we could consider 'phenomenal will' as resoluteness (Entschlossenheit), by which Dasein projects itself, seeking for truth (reality). Dasein does this by concern with the future — being futural — circling back on past experience and making decisions in the 'moment of vision': the present. Over and over again. A few indicative quotes here, (trusting I'm on the right track).
B&T page 388
In contradistinction to the moment of vision as the authentic Present,
we call the inauthentic Present "making present". Formally understood,
every Present is one which makes present, but not every Present has the
character of a 'moment of vision'.
and pages 414-415
Being which Objectifies and which is alongside the present-at-hand
within-the-world, is characterized by a distinctive kind of making-present.
This making-present is distinguished from the Present of circumspection in
that — above all — the kind of discovering which belongs to the science in
question awaits solely the discoveredness of the present-at-hand. This
awaiting of discoveredness has its existentiell basis in a resoluteness by
which Dasein projects itself towards its potentiality-for-Being in the 'truth'.
This projection is possible because Being-in-the-truth makes up a definite
way in which Dasein may exist.
The resoluteness which comes back to itself and hands itself down, then
becomes the repetition of a possibility of existence that has come down to us.
As historical, Dasein is possible only by reason of its temporality, and
temporality temporalizes itself in the ecstatico-horizonal unity of its
raptures. Dasein exists authentically as futural in resolutely disclosing a
possibility which it has chosen. Coming back resolutely to itself, it is, by
repetition, open for the 'monumental' possibilities of human existence.
As regards whether "a diminished sense of agency is inauthentic" — you could say diminished resoluteness is aways going to be authentic even if there is less of it, because what is proper to yourself is authentic. What is cultural and scientific, out in the world by contrast is 'ordinary' or inauthentic. (This is traditional terminology.) So if resoluteness' reality-seeking skills are diminished by impoverishment for instance, then you could say its agency is being diminished by inauthenticity, in the sense of the external situation.