Learning theory will open up A LOT of doors for your songwriting. You will be able to come up with musical ideas more consistently.

You have somewhat of an advantage because you play two instruments. Sometimes you can come up with cool musical phrases on another instrument that you would never have thought of on guitar. Piano also lends itself much more naturally to smooth voice leading (classical theory concept)

So in addition to everyone's advice, I'd recommend acquiring more theory knowledge it can only help.

Good luck,
Mike.