I put the focus on the second chord of G Major which is A Minor
Not only is
A minor the "second chord" (i.e., the
ii chord) of
G major, but
A Dorian is the "second mode" of
G major. That is, the
A Dorian and
G major scales have the same key signature. Put another way,
A dorian and
G major have exactly the same pitches in them.
Are there "compulsory" chord progressions in a Dorian Mode?
In order to make a mode sound "modal", one should simultaneously emphasize the unique pitches in that mode — the ones that differentiate it from other, similar modes — and deemphasize the similarities.
Dorian and (natural) minor are very similar, differing only by the sixth note in the scale:
A minor = A B C D E F G
A dorian = A B C D E F# G
More specifically, a scalar passage in minor might contain adjacent
G#, but not
G – the latter is unique to Dorian.
Thus, to create a "dorian" chord progression, it helps to emphasize the shift from
VI VII i and
III ii i will accomplish this while also clarifying the tonic pitch, which is also necessary to ground the mode. (For example, if you emphasize
G instead of
A, it's going to wind up sounding like
G major.) The progression mentioned in the OP, Am C D, is
i III IV in
A dorian and also prominently employs the
F# G shift.
i IV v also includes the tonic and the
F# G) point of emphasis, the progression overall is so characteristic of major and minor that should be used carefully, if not avoided.