MUST means omission is unacceptable. SHOULD means MUST unless you have a good, well-reasoned excuse.

Incorrect. Not required is not required. You do not need to supply rationale or get agreement by anyone else that your reasons are good in their opinion and not just in your opinion.

Should just means the thing is preferred. It's something that is good and useful and helpful to do.

That is not "must unless you can convince me that you should be excused".