Recently I experimented with Clang Tidy. I let it analyse some of my code and looked at the results. It was really helpful, because it suggested a lot of slight improvements that do not radically change your code. If you started with C and switched to C++ at some point, you will definitely still have some C habits in your code. Especially APIs expecting pointers to character buffers often use C-style arrays. There are ways to get rid of these language constructs, but you have to work on your habits. Avoiding C-style arrays is straightforward. You just have to do it. There is a reason why C and C++ are different programming languages.
Then there are the new C++ standards. In terms of security and convenience, the C++11 and C++17 standards are milestones. You should update your code to at least the C++11 standard. Clang Tidy can help you. There is a class of checks covering the C++ Core Guidelines. The guidelines are a compilation of recommendations to refactor your code into a version that is less error-prone and more maintainable. Keep in mind that you can never apply all rules and recommendations to your code base. Pick a set of five to seven issues and try to implement them. Repeat when you have time or need to change something in the code anyway.