If you want to do just one exercise, and you want to be able to do pull-ups any time soon, make it the pull-up. Imho, that would be the fastest way to go.
When doing Pull-ups, greasing the groove with single-reps (or if that's too hard, isometrics/negatives) is a very good way to go, at your level. Not just strength-wise but also in terms of improving technique and mobility (which may or may not be an issue in your case). If GTG is not an option, do as many sets as you can do with perfect form, 3-5 minute pauses in between - do this every other day or leave a couple of sets in the bank and do it +5 times a week.
If you want to do more than one exercise, I'd say train your Pull-ups at least as often as you train your chins, ideally 2/3 Pull-Ups and 1/3 chins just to maintain or even increase your chinning strength.
I had the opposite of your issue until about half a year ago: Trained the pull-up for years (15-20 pull-ups & 1rm with 32kg additional weight), NEVER trained chins, and I couldn't do more than 2 chins without pain. it was an issue of mobility and technique (grip width!). Did some mobility work for a couple of months, now my chins are suddenly stronger than my Pull-ups.