It sounds to me that both "department meeting" and
"departmental meeting" are correct. But which is

In fact, generally speaking, if the noun that
serves as an adjective actually has an adjective form,
it's better to use the adjective form?

E.g. "transition period" vs "transitional period"
"teenager problem" vs "teenage problem"

These days, people tend to use a noun as an adjective because it is allowed. To know which one is better to use, change the ' noun + noun ' to .... of ..... to make comparison with ' adj + noun '

department meeting = meeting of the department

departmental meeting = the meeting is regarding the ins and outs of the department.

Normally it is not wrong to use either one, because the meeting of the department is closely related to issues about the management of the department hah.