This isn’t aggression from your GSD. And if anyone should be corrected, it should be Cassie. She is resource guarding, both the food and the family. But since she is old and seems to be getting grumpier, I would crate the GSD when it’s Cassie’s turn to chill with the family, and crate Cassie when it’s the GSD’s turn. Feeding in separate rooms works, it’s what we used to do when it popped up. Now the can go around each other’s bowls without issue because they know I’ll handle it if one of them tries to eat from the others bowl. We still separate the pups from the older guys, and they are fed with distance between them, but they all eat at the same time just fine now.
As for your GSD starting to bite back, it’s a good idea to separate them, just because your senior is so much smaller. You’ll also need to supervise any time together they have together, and always be ready to step in when the size of your GSD gets to be too much for your small senior.
Lots of luck!