Based on my experience with my old cats over the years, there are some things you can do to make their sunset years as good as they can be for your old friend.
The eschewing of dry food for soft could be due to dental issues. Since you're already seeing a vet about this, they would have mentioned this to you if it were a problem. Also, very old cats, especially those with other problems, aren't good candidates for dental procedures because they require anesthesia. In any case, at this stage of life, the important thing is to keep them eating, so give them whatever they want.
For failing kidneys, the usual methods of treatment can include a special "kidney diet" food (available in both soft and hard,) phosphorus binders (if phosphorous is high which is frequently the case in kidney kitties,) and the periodic administration of subcutaneous fluids (has a kind of "dialysis" effect by helping flush the kidneys.) The last item can be done by the vet from time to time (maybe every week or two) or you can do it yourself (it's pretty easy to do and the vet or a vet tech can show you how.)
Also, there are appetite stimulants that may help, but it sounds that you don't need those.
The vomiting may be reduced or relieved by the daily administration of a quarter of a Pepcid tablet (dosage depends on weight), which reduces nausea and acid in the stomach. Again, the vet will advise you about this.
The hypothyroid condition is treated the same way as it is in every mammal - Synthroid (levothyroxine.) Dosage depends on weight, it's very safe and inexpensive. Your cat's reaction to it may have been due to the dosage or his kidneys. Allergic reactions to it are very rare.
In any case, your vet is the best source of guidance on all of this. You should talk to them about what I've said before proceeding with any of it. They see similar kitties almost every day and are very familiar with what would be appropriate here. Eventually, your old guy's time will just run out. That fate awaits us all. But you can still do things to make your pal comfortable and happy for the time he has left. That's all any of us can hope for.
Good luck to you and your old devoted friend.
= new reply since forum marked as read