Please don't be too hard on yourself.I know I'm tough on myself and I keep wondering if I missed any signs of his illness/condition through lack of awareness.
The fact is that the onset of the acute thrombotic obstruction is very sudden, and often there are no prior signs, unless the cat was diagnosed with cardiomyopathy prior to it. It's true that there are certain breeds that are predisposed to it, but often it's hard to predict. Only if you do regular annual checks, including echocardiography. And this sometimes hard to do due to lack of funds, or not necessary unless your cat has symptoms of cardiomyopathy.
Your cat is a mixed breed, so there was no good reson to suspect that he could get something like this.