It impossible to say whether this is DIY case or not because we do not know your life history, skeletons, or things you are forgetting to mention, and so forth. Plus things are a lot tighter and stricter now. You can't listen to or compare your case to others when you don't all the facts of each case.
But if you think there are things that might be an issue, then it makes sense to get a good lawyer to avoid "little things" that might cause you trouble. People do get denied for lots of reasons that could have been avoided, and in some cases not be avoided.
But if you know if your case is very straightforward, nothing unusual, no criminal history, drug use, no arrests, etc etc .. and are simply paranoid about doing the paperwork correctly, and want to sleep better at night, you could compromise and get an agency instead of a lawyer to help.
Otherwise, it is true that the N-400 is very simple. And much of the evidence you used for the ROC can be used here. Just do it yourself, its not hard, don't over think it. I would agree that DIY would be fine.
Bottom line, whether its a lawyer, agency, DIY, etc.. depends on a million variables that only you know. I would suggest you ask your lawyer if you have any issues that would complicate your specific case. According to google 10% of people are denied the N-400. If you have no risk factors and can easily fall into the 90% you can DIY, or get an agency to help you, If you have legal issues that might make you not eligible, probably better to get a lawyer