Marius is spot on with his question.
A high degree of Instrument proficiency is not necessary to be a versatile composer. A versatile composer is one who may write for two or more situations: concert and dramatic music for traditional instruments, electro-acoustic, educational music geared for church use ot pedagogical use, and sound design ( usually this may mean creating more realistic renderings of other people's computer renditions of their music, creating sound effects and meshing them with composed music - common with film and video game music, working with visual artists and architects to create sound component of a multi-media installation).
In addition, a versatile composer will possibly teach or engrave other peoples' music
However, if you wish to be a composer and earn your main income from music, then your best bet for income is composer/performer/teacher where advanced instrument proficiency is a huge help.
What is most important is EXPOSURE to music and developing your ear through singing and aural drills. This is just for building a disciplined awareness so you get the most from your wide exposure to a variety of musics. Counterpoint studies I advocate mainly as it is a centuries-old procedure to develop musical discipline. Plus it has general utility as one tool for judging the spacing and relations of your voices.
The question you have to ask in the future is: are you committed enough to composition and earning your income from it so that you will do some of the following?:
1) Teach a general music course to k- 6th grades as a visiting composer or general music teacher? Or go on the college teaching track by teaching a few classes as an adjunct, lead say a choir for extra money, while you have private students and earn your doctorate.Not many get tenured college teaching positions.
2) Develop a new music group to perform your works - that is find funding, get musicians, pay musicians, find venues, etc???
3) Write music for films or commercials that you have to generate enthusiasm and interest in when starting out so you get projects that interest you.
I think to start you need to write for a number of ensembles, solos, etc and a variety of purposes (a concert, film, choir, etc) to find what you want to do.
The one caution is, if composing ALWAYS becomes something you HAVE to do and you are indifferent to the results because you are so concerned about paying bills or "making it big" , then I would pause and reflect why you are even doing this as there are more lucrative ways to pay the bills, save, and take a vacation than writing music.
Finally, check maestrowick's post which further expound on these topics.