I'm so sorry for your loss
.
It's hard to say what went wrong for sure. Anytime at all there is a breathing issue of any kind, a dog should go to the vet ASAP - so it's really odd that your Vet didn't have you come in immediately with all of those super concerning issues. Inappetance in dogs is another cardinal sign that something is for sure wrong - dogs who eat normally and then suddenly stop doing that are telling you clearly that something has changed and is wrong.
Was he outside at all and could he have eaten anything (ie a Sago Palm nut or leaf, something else toxic, lick antifreeze, etc)...? Do you have spiders or other bugs in the area who have toxic stings...?
It also could've been heart/lung related - like a pulmonary embolism, pulmonary edema secondary to CHF (unusual for his age), pleural effusion (but why, I don't know).
Lastly, also could've very well been a GI blockage - which absolutely will cause death if untreated. You said he was constipated - when did he last go..? And his symptoms (rapid breathing, struggling to eat, shaking...getting worse on Tuesday with being so weak etc) are all evident of pain signals and finally weakness as the body is starting to shut down from the blockage.
Did he have a chew recently where he could've consumed that last big chunk of chew...? Did he eat rocks or anything outside that you noticed before...? Eat from the garbage ever..?
Again, so sorry. Those are the things I think of with what you described. It does not sound like a stroke or heart attack in the beginning to me.