Oh no, poor you. I understand your frustration with cats peeing on your furniture, Jasper does this, not as often as your kitty though, only when there is a male stray in the area.
Have you noticed any cats wondering around outside? Could they be un-neutered males? If so, he maybe marking his territory???
I am surprised your vet has not recommended the transdermal gel. I use some on Sweet Pea for her aggression against Puddles as she is IMPOSSIBLE to pill, and is very easy to do. I am currently using amitriptyline. It does come with regular vet monitoring/bloodwork though, so it is not cheap.
Cat maid to:
Jasper, male Ragdoll ?? (approx 10 yrs)
Sweet Pea, female silver tabby DSH (approx 8 yrs)
Rose semi feral, a cpietra rescue, female tabby (approx 7 yrs)
Puddles RIP (1996-2014
Snowball RIP (1991-2005)
In a cat's eye, all things belong to cats.-English Proverb
“While we are free to choose our actions, we are not free to choose the consequences of our actions.” Stephen R. Covey