Every type system is eventually used at five to six by someone who has not read the guidelines and will not start now.
Systems fail the same way. A scale of nine sizes is specified, three get used, and the other six are improvised in a deck by someone who needed something between two of them. Within a year the scale exists only in the document.
Fewer sizes than you want
Four sizes used correctly beat nine used approximately. Each should be far enough from the next that choosing between them is obvious rather than a matter of taste.
If two sizes are close enough to argue about, one of them is unnecessary.
Four sizes used correctly beat nine used approximately.
Bind the size to a job
Name sizes after what they do, not how big they are. Specification, body, label, display. A junior choosing type then answers a question about content rather than a question about scale.
On the sensor company this collapsed three datasheet templates into one, because the fields carried their own type decisions.
Make the wrong choice hard
Ship the system as styles in the tool people actually work in, not as a PDF. If the only reachable sizes are the correct ones, the system holds without anyone policing it.
Documentation is a backup for a system that is already enforced by the file.