I noticed she is on Previcox, and that can cause GI symptoms. You might want to look up Previcox in dogs and see if you think that has anything to do with it?
You mentioned previously that you think the digestive issue might partly be caused by things not getting moved through her system as well due to inactivity. I would think some added fiber might help with that. My down dogs have all done really well with Science Diet w/d dry kibble. The canned variety of w/d did not work as well. With the w/d dry they have good stools that pass through as they should, and they are not soft and do not make a mess in your pet's bed, the carpet, or your pet's coat. It's been a godsend. Personally I do not recommend just adding fiber sprinkled into the regular diet. I know of a dog who bloated with just a small amount of added fiber, though I know it is sometimes recommended. I guess you could ask your vet. But if you want to try to make the stools able to transit her system in a more regular way by adding a little more fiber and low fat in the diet, the w/d would be a good thing to try. If it does not work for your dog, Science Diet used to take back the bag and refund your money. I don't know if they still have that guarantee, you could ask the front desk staff at your vet and find out for sure. Also, some vet clinics will give you a ziploc bag with some to try instead of buying a whole bag. It's prescription so you have to get it from the vet.
Since your dog is experiencing digestive issues and has responded positively to Tylen in the past, it makes sense that your veterinarian prescribed it as a long-term solution. However, it is always a good idea to discuss any concerns or possible options with your veterinarian.
I don't have direct experience with Tylan, but I can understand your concern about having your dog on antibiotics indefinitely. It's a valid worry. Long-term antibiotic use can sometimes have its own set of considerations.
Have you had a chance to discuss your concerns with the vet? They might be able to provide more insights into the long-term effects and potential alternatives. Sometimes, they can adjust the treatment plan or suggest additional measures to address the digestive issues without relying solely on antibiotics.
On a personal note, a friend of mine had a senior dog with digestive issues, and they found that incorporating a bit of plain, unsweetened yogurt into the dog's diet helped with gut health. Of course, it's always good to run any dietary changes by the vet.
Sending positive vibes to you and your pup. It's clear you're doing your best to make sure she's comfortable and happy. If you ever want to share more or just chat about your furry friend, I'm here!