Another interesting case of a composer who didn't know how to play the instruments they wrote for is Joaquín Rodrigo; he wrote the most emblematic piece for classical guitar, El Concierto De Aranjuez, without knowing how to play guitar (he was a pianist). Shows you that it's all about understanding the language of music and the instrument. I agree with your choice of instruments for composing. Being able to sing the melody is crucial, since it means that you understand it and are able to reproduce it. Guitar, as you said, is great for sketching songs, and the piano is great for arranging. I personally like composing with a guitar; once I have something I like, I record a bass that sounds interesting against the guitar part. Then add extra parts or arrangements with keyboard. But sometimes I start with a bass part, or a keyboard or drum part, it really depends where I got the idea first. As you say, a lot of this really depends on the genre of music you're writing for, but knowing how to build chords and melodies on piano and guitar is really helpful. Great video!