Most useful single nutrition rule: eat mostly real (minimally processed) foods. The distinction between whole foods and ultra-processed foods matters more than most specific dietary rules (low-carb, vegan, etc.).
What counts as 'real food'
Foods you could cook yourself from recognisable ingredients. Vegetables, fruits, whole grains, meat, fish, eggs, dairy, legumes, nuts, seeds. Single-ingredient foods primarily.
What to limit
Ultra-processed foods. Items with long ingredient lists you wouldn't recognise. 'Healthy' processed foods that are still processed.
Simple rule that addresses most modern dietary problems without requiring detailed knowledge of specific nutrients.