Cat with diarrhea no matter what vet tries?

My cat is 15 years old. She's always been an easy cat, had one instance of ear mites a couple of years ago, and then another instance where she had scratched some of the fur off the tops of her ears due to stress when I was away for a month (I still lived with my parents, she wasn't in an unfamiliar space, she's just extremely clingy).

Now, nearly 2 years ago, I moved out and took her with me. For the first while, everything was fine. She was eating a little less of her kibble and I started giving her wet food every morning. That was fine for nearly a year, before I had to go to the vet because she was having diarrhea and throwing up every night. She got put on some special kibble for GI issues, and things were looking up since.

Since then, she's had an instance where she threw up blood once, only for it to never happen again and for the vet to say that it's nothing to worry about and it must've nicked something in her throat (it was mucus with a pink-ish tone). Another issue where she has a possible URI, scratched her eye open and was put on some ointment. Then another where she started walking a bit strange, so she had to take anti-inflammatory.

Now she has diarrhea again and is getting Vitamin B12 shots at the vet - the plan is one on week one, another on week 2, and another on week 4, to then see the progress.

She has had blood work done in the past that all came back fine, nothing out of the ordinary. She eats and drinks just fine, behaves the same, and still uses the litter box (aside from peeing on my bed or couch when I've been away from home for too long - she deems over 10h a day too long).

I don't really know what to do. An echo/ultrasound wasn't advised since they'd have to put her to sleep if I understood correctly, and they said at her age there was a chance she wouldn't wake back up. I'm hoping the B12 works eventually, but I'm looking if people had something similar happen and what they did. I just want her to not be in pain.

* Species: Cat * Age: 15 * Sex/Neuter status: Female, spayed * Breed: Regular housecat * Body weight: 3kg * History: in post * Clinical signs: in post * Duration: less than a year, more than 6 months * Your general location: Belgium