In my case, the heat gets to the living area, but it's never as warm
as in the bedrooms. In summer, the situation is reversed, except it's
worse: hot living area, cool bedrooms.

If this is consistant, then
that just means that you have to arrange for more air to be
delivered to and returned from the living area, and less to
the bedrooms, until it's right.

You'd only need a separate heater or zone if either the
error, or the desired relative temperature of the assorted
spaces changed, at different times.