You are probably correct that you are stagnating. I've been there, and was happy with it for a while. But when it was time to leave, I had to work hard for about a year to skill up and prove that I could do modern tech stacks. It was a pain, but totally worth the effort.

At the same time, that salary is not bad for a lower cost of living area, so if it is stable and low-stress, that doesn't sound too bad.

I'd say that the question is whether or not growing as an engineer is a goal in your life, or if you are happy just working at your decent job, and then living your own life outside of work?