Unfortunately I don’t believe that’s enough. For things like android/app/build.gradle there’s no nice parser/serializer to make updates, so you have to basically do a search and replace to make the changes. That’s why it’s classified as a “dangerous” mod.
I haven’t written any config plugins in quite a while and I never considered myself an expert, but what I found helpful in the past was searching Expo’s repos on GitHub for references to e.g. appBuildGradle
In order to use your own plugin, you can just dump it in a file in your app (it can be called anything), and then reference it in the plugins section of app.json like this: