We are in a similar position OP, I really feel for you.
My boy is approaching 16. He has odd neuro type episodes, which may or may not be partial seizures (vet not sure). He has well managed arthritis and old dog vestibular disorder, which flares up from time to time. His back end is getting weaker, he’s wobbly, doesn’t do stairs, but can get up and down the back door step. He knuckles over on one hind leg intermittently, but does still right it and will if you do the planting reflex test as well. He’s recently had full bloods done and they were all ok.
We have good insurance and have discussed referral to specialist neurologists with our vet, but, given they don’t want to do anything about his one dodgy tooth, as the feels GA would be unwise, there’s no way we would put him through an MRI or any other tests. So we’re trying to capture video of his episodes and she is going to use those to consult with them for ‘best advice’ for management instead.
Meanwhile, he’s happy, still loves his food, brain training, snuffle and licki mats and going for short pootley/sniffy walks. He sleeps a lot and isn’t interested in his toys anymore, but boy can he shift when he wants to - usually when he smells dh making his afternoon apple and peanut butter snack! He’s a bit deaf and doesn’t hear when we get his food bowl out anymore, but if he smells food being prepared in the kitchen or the scent of food in the oven he’s in there like a shot. He likes his routine, we are clearly told what he expects when and he doesn’t stand for any delays (he does top class Paddington stares). He still has a geriatric version of the zoomies after his dinner every evening and likes to play a game where he goes in and out the back door, while one of use ‘chases’ him. Then he stands on the lawn and waits, till we get just within reach of him and off he goes at his geriatric zoomie pace, back indoors to start the loop again. We have to do that four times every evening. After that he’s always exhausted, but super happy. He also still likes to sneak up to the magpies and pigeons on the fence and do one great big ‘woof’ to scare them off. So despite his struggles, he still has a decent quality of life and happy and enjoying himself.
So, our plan is to keep doing everything we can to help him remain happy and comfortable for as long as we can. Keep monitoring him, regular vet checks (currently see them monthly) and keeping a check on his quality of life and if things change for the worse, let him go before he starts to suffer.
It really is the hardest thing to be going through. We lost our other boy relatively young (two weeks after his 7th birthday) this time last year. He had been ill for a long time with a horrible auto-immune condition, but the end was at home, sudden and a shock. I really couldn’t go through that again, even though actually, he wouldn’t have have known anything about it. I definitely want to be able to give my old boy a peaceful end to a happy life and will admit to being constantly hypervigilant for any signs that we need to be making tough decisions. I can only hope we are doing the right thing by him.