I live in central NY and I think you should bring your attic up to an
R38. This can be done by either fiberglass or cellulose blown in
insulation.

Is your garage wall colder than other exterior walls? It shouldn't be
any worse than other exterior walls if you keep your garage door
closed. You may think this is a silly statement but I've heard a few
insulators grumble because they get calls from disgruntled homeowners
but when they check on the situation they find that many of them leave
their garage doors open for extended periods of time.

If you install Dow Styrofoam or Dow Tuff R both must be covered by 5/8
type x drywall for two reasons. 1) Both products have high flame
spread/smoke developed characteristics and must be covered by code. 2)
Your garage wall is probably a one hour fire rated wall and will lose
that rating if it doesn't have firecode drywall on both sides of the
wall.