Flexing once it heats up makes me think epoxy may not be strong enough to hold the patch. If it is getting that thin, perhaps taking it to a competent welder would be a better choice. The repair may cost a bit more, but if you like the smoker, this type of a repair will last much longer.
Believe it or not, smokers do wear out. Some are built heavy enough they will last several generations. But, they are the exception, not the rule.
My other thought is to put your "hunter-gatherer" hat on and begin stalking a replacement smoker. Doing all the homework and searching for the next "big" purchase is very enjoyable (to me anyway).
Let us know how it turns out.