I'm fairly familiar with this, as I have an older Akita who is going thru the same thing, multiple reoccuring UTI's.
As they age, the bladder sphincter becomes weak. They may leak a bit while sleeping, or become incontinent altogether. When this happens, bacteria can enter the bladder much more easily. The urethral opening in a dog is located internally, in the vestibule, where urethra and vagina join. That area normally contains bacteria anyway. (not E coli)
I have had to resort to antibiotics a couple of times, but what actually improved her was Acupuncture and expressing her bladder regularly. The Acupuncture strengthened her bladder muscle and sphincter, forming a better barrier to bacteria. Expressing her bladder, a very easy thing to learn and do in a female dog, helps to keep her bladder empty. Stale urine grows bacteria faster.
Herbs I have tried at different times are; Usnea, Yarrow (when I saw blood), Cleavers, and Poke. None have been successful at clearing an infection completely, so I've had to use antibiotics, then work to repopulate her gut bacteria with probiotics.
Some other things I do ongoing; Vitamin C 2000 mg daily when not infected, up to 4000 when she is. I found a brand of C that is 1000 mg and is time released, which I feel helps. She weighs 70#. Cranberry capsules twice daily (1680 mg). Fish oil caps, two daily, helps with inflammation. No carbs in diet. Zero, nada. Carbs feed bacteria. (I feed a raw diet anyway)
You might consider asking your Vet to show you how to express her bladder. Most dogs don't mind it at all.
As for your current course of treatment, if it's working I'd continue it ongoing, unless you see problems associated with it. I'm not familiar with Oregon grape. Keep up with the C.
Hugs to your old girl. I love the seniors.