Writing a string quartet is a goal of mine too. I can't advise from a background of finished quartets, but I can share what I've tried to get there.
But first, I think don't keep trying to adapt chorales. Certainly you can play chorales with a string quartet, and it sounds nice, but the texture is different than the typical string quartet. For example, in quartets you will have a lot of broken chord, broken third, repeated note accompaniment patterns that never appear in a chorale.
Another big difference is form. A chorale is usually a few phrases that modulate quickly to related keys. There usually isn't a lot of repeated and varied phrase structure which is what you will find in a quartet. In a quartet you will commonly have sonata, minuet da capo, and rondo forms. Periodic phrases and recapitulation will be important structural devices.
This is what I have tried:
- write minuets first (16 bars), the work up to the trio with da capo repeat. Structurally this is the smallest part of the quartet. If it can't be handled, the other forms will probably be too much. Try writing "sketches" rather than working out completely the final, four parts to shift the focus onto the formal structure which is the real concern of composition.
- write short phrases of complete four part settings with attention to the distribution of melodic elements among the four instruments. I used Mozart for my exercise. What I saw frequently is the main melodic interest is often in one or two voices with the others playing very simple accompaniments. Melodic stuff can often be doubled in two parts in thirds or octaves.
- Copy quartet scores into a notation program. This may seems like a waste of time, because your aren't making any compositional choices. But, when you copy note for note from the score - which is very tedious! - you force yourself to be much more aware of the contents of the composition. I did this with the opening of Ravel's string quartet and saw things I hadn't really paid attention to before when only reading/listening to the score. I then tried writing a quartet opening of my own modeled on the features I saw in Ravel's.
- Do analysis of quartets and write harmonic reductions. You should find that the harmonic phrasing and harmonic rhythm is simpler in a classical quartet than in a chorale. Very broadly speaking four bars of a chorale will have lots of harmonic movement whereas four bars of a string quartet will sometimes be not more than tonic/dominant harmony. Texture-wise a chorale is more contrapuntal, a quartet more homophonic.
I'm thinking mostly of Haydn and Mozart for the quartet models and comparison to chorales. A Romantic era composer like Brahms might provide better models for expanding a chorale texture to a quartet.
I had one other thought. If you, like myself, are not a string player, try to familiarize yourself with the string instruments. Get a cheap violin if you can, or a mandolin which has the same string tuning an neck scale. Knowing about how the bow feels, fingerings, etc. puts you in better touch with how to write for strings. Connect with anyone who plays strings for advice or just for super close listening. I had a friend in college who played cello. I would pester him to play the Bach cello suites. I could site right next to him, read the score along with his playing. I tried to write little things in the same vein and he would play them... such as they were.