You can move from ear-learnt Dutch to an academic basis by systematically studying formal grammar rules and then consciously applying them to your existing intuitive knowledge. Start by identifying the specific grammar structures you already use unconsciously, then learn the rules behind them to gain control and precision. This approach lets you keep your fluency while adding the depth and accuracy you want.
First, take an inventory of what you already know. Write down a few sentences you would naturally say, then look up the grammar behind each part: word order, verb conjugations, case usage, and sentence connectors. This connects your ear-learnt patterns to explicit rules. For example, if you often say "Ik heb gisteren een boek gelezen," study why the past participle goes at the end and how the auxiliary verb works.
Next, use a structured grammar reference. Look for a comprehensive Dutch grammar book or online resource that covers topics in order: basic sentence structure, verb tenses, modal verbs, word order in subordinate clauses, and then more advanced topics like the passive voice or the use of "er." Work through one chapter per week, doing the exercises. The key is to not just read but to write your own example sentences and compare them to the rules.
A honest tradeoff: this process will feel slow and may temporarily make you more self-conscious in speech. That small voice saying "maybe this is wrong" is actually a sign of progress. Embrace it. The goal is not to replace your intuition but to refine it. Over time, the rules become second nature again, but now with a solid foundation.
Concrete next steps: 1) Choose one grammar topic per week (e.g., word order in main vs. subordinate clauses). 2) Each day, write 5 sentences using that rule. 3) Read them aloud and check against a grammar guide. 4) After a week, try to use that structure in real conversation or writing. 5) Gradually layer on more complex rules like the use of "om...te" or conditional sentences.
Finally, seek out materials that explicitly label grammar. Look for textbooks aimed at intermediate learners (B1-B2 level) that have clear grammar explanations and exercises. Avoid purely immersive resources for now. The goal is to bridge the gap between your ear and your mind, not to start from zero.