Given Jasper's fear of the vet, the fact it's nigh-on impossible to inject directly into a vein, and he fights sedatives all the way, I have wondered about options... At some point I'll need to raise it with my vet.
Judy,
I have twice assisted with dogs that had to be euthanized due to aggression that stemmed from fear (one was legal based, the other safety), in both cases the dogs FOUGHT the initial sedative. In hindsight the mistake was treating the initial sedative as if the dog was just needing it temporarily.
While not being a vet I can't give medication advice, but I would suggest talking with the vet about just really "going for it" with the sedative. don't hold back vs habitually following dog is x weight so that is the correct dose.
Another tip, if you are "lucky" enough to be able to plan how Jasper passes, and you know he will fight the sedative, is to be sure that you keep his last 24 hours as low key as possible. make sure he isn't amped up with adrenalin for when the time comes.
I am hoping Jasper just passes in his sleep when the time comes, that is how Jack went. went to sleep, didn't wake up.