Routine dental X-rays are not backed by evidence

Some dentist practices (maybe they are chains) do seem very shady when it comes to overtreatment. I remember on my first visit to an office that I was recommended customized trays that I could wear overnight to have my teeth/gums soaked in hydrogen peroxide gel. This recommendation felt like a sales pitch and when I researched the proposed treatment code later I started to find some dentists online claiming that they wouldn’t recommend those because they are not clinically proven to work against gum disease.

I understand that radiation effects are cumulative but is this overexposure source worth fighting against as a patient?

Right, that's really the problem: that question is impossible to answer in general because presumably the person who knows the best is the professional who actually examined you. And if you dare question him he's going to be offended.