This is a very good question, I have parts of the answer, but this answer will not be conclusive. The dietary requirements of livestock rabbits has been the subject of much research, but as you correctly point out, the requirements of pet rabbit are different and not as much research exists for the pet population.
You have made a good choice in limiting carrots and tomatoes which are not a healthy choice think of them more like candy, occasionally OK in moderation.
The primary question here is the appropriateness of kangkong for long term diet. The first place I looked was the Suggested Vegetables list at rabbit.org which does not provide an answer but does indicate the difference between leafy greens in the suggested diet is the amount of oxalic acid high levels can be problematic in for long term use, with kidney stones being the concern. Spinach is notably high in Oxalic acid, but "water spinach" is not a member of the spinach family.
So what is the 'Oxalic acid' content of 'Ipomoea aquatica' (kangkong)? There are lots of answers to that question 1, 2, 3, 4, and I don't find a reliable reference that clearly defines it. Some sources say high, some say low, some don't say. So pending a clear reliable reference the 'Oxalic acid' content of 'Ipomoea aquatica' should be considered potentially high.
Is 'Ipomoea aquatica' (kangkong) an acceptable substitute for hay? Hay is a critical part of a rabbits diet the two primary reasons being to keep the ever growing teeth worn down, and to assist in moving hair through the digestive tract to prevent hairballs from being problematic. While both of these issues are potentially life threatening they are not acute, they develop slowly over time, so regular visits to a qualified rabbit vet should be sufficient for monitoring for these symptoms.
While not definitive there are potential risks, you should be concerned. There are potential long term impacts, with kidney stones being the highest risk of sudden onset. Sudden diet changes can be fatal and would be of much greater concern than the risks of the current diet. Hay should be available 24 hours a day, with leafy greens feed 1 to 3 times daily. You can begin transitioning over a couple of weeks to a cost effective choice from the Suggested Vegetables list at rabbit.org in our family, romaine lettuce is the primary leafy green choice. With hay always available your bunny should begin to eat it between meals, and it should become a staple part of both keeping their belly full and an enjoyable daily activity.