My cat's tumor was at the base of her tongue and, prevented her from eating, a fact we didn't know, because we were also treating her for kidney disease, giving her Sub Q fluids every other day......sadly, we learned that was the only fluid she was getting.......a fact that has me wracked with guilt (I had no idea she couldn't drink, I'd see her at the water bowl several times a day, appearing to be drinking) she was eating, very little but, I associated it with the kidney disease.
That's what you do, you second guess yourself......even though, you know you did everything you could.
I'm sure your Smixy knows he was loved.