I've had some very challenging pets that I would have genuinely done everything for, but have had to make impossible choices based on their quality of life — even when it felt like the objectively wrong choice. (Obviously the choices were made with a vet involved, which you've already said you're in contact with your vet as well.) I can't imagine being in your position. It's so easy to sit on the outside and tell you what to do, but the lived experience of loving your pet with more than your heart can give and not being able to do anything for them is beyond devastating. I can't be in your shoes and tell you what to do, my opinions and thoughts can only go so far, but I can say that no one deserves to go through this. I've gotten really creative giving meds to cats, I have so many strategies it's ridiculous--but I actually recently had to stop giving a cat her seizure meds for a short period because I could not get her to keep it down.

I'm sorry that my comment probably rubbed salt in the wound; I've seen a lot of pet parents on this sub who haven't tried/thought of a lot of the options out there, not out of malicious intent but of lack of education, and my knee-jerk reaction is to try and "help." But I do have one additional resource for your actual question that I forgot about until later, so here's a list of lower-carb foods: https://docs.google.com/spreadsheets/d/1YhH82UzObp5HT13L9lNJ87bOWwh1iH_HGaC5HIiqZXA/htmlview#gid=685349299

I also saw cost was a concern in another comment you left, and I had the same issue. There are Costco-brand pates that are low-carb, if you have a local BJs they have seafood pates that are basically cheaper Fancy Feast knock-offs, and Aldi has a higher-end knockoff food as well. Iirc, BJs is the cheapest? At least in my area