Since your pony is 33 years old and most likely a companion and not active, I would suggest the Integrity Lite without molasses which has 1.6 % starch.
There is much confusion in the horse industry about carbohydrates which is understandable because carbohydrates are a complicated class of chemicals. There are different laboratory assessments used to measure the different classes of carbohydrate chemicals. A standard to provide an understanding of starch content in horse feeds is the numerical value that represents % starch and the ethanol soluble carbohydrates (ESC). The ESC for Integrity Lite without molasses for is 5.9% thus the %Starch + %ESC for Integrity Lite without molasses is 7.5%.
Integrity Lite without molasses of course does not have molasses, does not contain any grains and the first two ingredients are beet pulp and soybean hulls which are fiber sources that promote gut integrity. You indicated your pony has worn-down molars and has issues with chewing so adding water to soften the feed is a practice some horse owners will use. Start with equal volume of water and the goal is to soften the feed. If your pony is an aggressive eater and tends to eat fast and not take time to chew, you can place small rock boulders in the feed bucket to create obstacles so that he has to navigate around the heavy objects to eat. This nutritional management practice will slow down the rate of feed consumption. Obviously, the small rock boulders phrase is a bit-of-humor because the rock-obstacles must be large and heavy enough so that the pony or horse is unable to pick any up by the mouth.
The Integrity Lite contains not more than 20% crude fiber but I always recommend a fiber source such as baled-hay or pasture. If baled-hay (long-stem fiber source) is not an option then you can feed a hay pellet. Fiber is a critical part of the horse’s diet so adding the hay pellet with the Integrity Lite is a better option than a ‘complete’ feed. You will have flexibility in regulating his diet.
Follow-up Note: Further details about the pony (see What to Feed your Horse) provided the information needed to recommend a blend of 1 lb. Integrity Lite without molasses with 6.5 lbs. of a hay pellet to start; adjustments would be made based on body condition score changes.