Oh my gosh, I'm SO sorry you and Kitty are going through this! My heart goes out to you
. I haven't had to deal with a cancer diagnosis in a cat, but I do know of other pet owners that have gone the chemo route for lymphoma. Cats usually tolerate chemo quite well (much better than people do). The other thing with at least trying it is that if the side effects do become too much to bear, you can always stop. As long as Kitty is okay with the process, it might give you a little extra time together without too much stress.
You're on the right track with her diet. Definitely low-carb (cancer cells feed off of carbs), and good quality ingredients. Maybe add some raw food in there as well if she'll go for it. I like to rotate brands and flavours of canned to keep the cats interested and to prevent any possible nutritional excesses or deficiencies.
As for supplements, you might want to look into something called Transfer Factor. It's derived from colostrum and can boost immune system health. The vet might also be able to suggest some things, like perhaps a multivitamin (I like NuCat).
And of course, the most important factor in all of this: your unconditional love. It's easy to get obsessed with a sick kitty's health and to focus too much on every little sniffle and sneeze. Try to take a step back now and then and just love on her, cuddle her silly, and appreciate her for the life lessons she's teaching you during this difficult time. Think of it as an amazing journey the two of you can share together.
All the best.