One more vote for finding a piano teacher who can help you learn theory in a classical sense. In addition to classical music theory my son’s piano teacher also taught him to play chords from chord (lead) sheets like a popular guitar player would use. The practical skill of understanding how chords support a melody and how to play that has been very useful for him. He’s done some composing for piano and for strings. If you want to do composing for multiple instruments it will be helpful to explore the capabilities of the types of instruments you will be composing for. Maybe volunteer in an elementary or middle school school band or orchestra program so that you can listen in and observe.
I think it’s great that you want to do this. Both my husband and I have taken up new instruments in the last few years as our kids have gotten older. He practices more than I do, and we may never perform on a stage, but we are both learning, making progress, and enjoying ourselves. I learned a ton about music sitting and waiting through my kids’ many many many music lessons. It’s fun to try to apply some of it!