I've never understood the difference between Bearing and Course to steer.
See page 112 of your user manual.
As far as I can deduce:
Bearing is the track from your present position to the desired waypoint
Course is Spam-speak for 'Desired Track' - i.e. the track you'd planned to be on between the waypoint behind you and the next waypoint.
Course to steer is a recommended correction to recover to DTK by reducing track error, although I cannot find a good definition of precisely how Garmin compute this value. It is supposed to give you an 'efficient' turn demand, whereas 'TRN' is TRK-BRG, the angle to turn through to track
direct to the desired waypoint. My guess is that Garmin continually generates a pseudo-waypoint somewhere ahead on your planned track and gives a turn demand to intercept it - perhaps so that the the triangle formed by this pseudo-waypoint, the next waypoint and your present position is an isosceles triangle, with the equal sides being from present position to pseudo-waypoint and from pseudo waypoint to next waypoint; thus CTS steadily reduces as you regain desired track?