I think depth!
I would structure this as:
1. Why CMS (100 words): Pick 1 reason (be specific! use evidence / example if appropriate) --> Why is this important to you?
2. Qualities, skills, experiences (100 words): Pick 1 - 2 skills (ideally ones that CMS targets) --> Use 1 - 2 experiences to explain how you developed each skill --> How could you apply this at CMS?
If you're struggling with word count, you can prompt AI to 'reword to be as concise and easy to read as possible' (but check that CMS permits AI use).
Best of luck!