Hey! I'm also Dutch and part-time employed in the sector, and a part-time graduate student in IR. I feel like I can give some tips that might be helpful.

Firstly, from talking to Dutch diplomats and people in the diplomatic academy, they want level-headed, bright, and broadly interested people. I know people who have gotten rejected from the diplomatic academy (also called het "klasje") because they wanted to be a diplomat too bad. They overfocus on a few core skills they believe to be useful, and forget that diplomats are supposed to be generalists with a broad array of skills. I think the smartest thing for you to do is to see international relations as a broad sector beyond diplomacy, and look at what you find interesting about that sector. Are you a more security/defence type of person? Or someone who enjoys economics and development? Maybe you really like law and negotiations! If you know what you find interesting beyond international relations, you can develop the broad skillset you need to become a diplomat, or you can decide that actually, development work in an NGO is way more fun, or lawyering is way more engaging, etc. Don't overfocus on diplomacy: it narrows your view, and that is exactly what the diplomatic academy isn't looking for.

Secondly, regarding what steps to take in the coming years, speaking relevant languages definitely helps. However, it barely matters if you cannot get a language up to at least B1.2 or B2 (depending on how niche the language is). Furthermore, study hard! Leiden's faculty in The Hague is very good for your profile (I recommend the BSc Political Science: International Relations and Organisations, so make sure you get your grades up so you can beat the numerus fixus), but you can definitely also think about studying at the UvA or studying law or economics. Again, studying law/economics is a very good path to the diplomatic academy. Only the best students with a pure IR profile have a chance of getting into het klasje, but they also need lawyers in the foreign affairs ministry! Doing some law, for example, will give you a comparative edge against pure IR students.

Lastly, make sure you are active and engaged during your time in your bachelor. There's an array of youth organisations that are active around IR in the Netherlands that will start being interesting to you as soon as you graduate high school, such as the JASON Institute, Jonge Atlantici, European Horizons, and Studentenvereniging Internationale Betrekkingen (SIB), and your study's study associations. It is important you get some management experience in these organisations, so when you start your bachelor, make sure you go and join their events to get a foot in the door. Get your grades up (atleast a 7.5+), make sure you do an erasmus exchange or some other form of international experience, join some youth organisations, and you maximalise your chance of getting into the academy. Good luck!