The difference between a "good" English speaker and a "fluent" English speaker is often just 365 idioms. Grammar gets you to 70% fluency. Vocabulary gets you to 80%. But idioms, slang, and cultural expressions take you to 100%.
Use idioms sparingly and contextually.
Learning idioms in a "one-a-day" format is highly effective for long-term retention. Resources like the 365 American English Idioms by Michael DiGiacomo or Jackie Bolen's idiom guides are designed to prevent learners from feeling overwhelmed. 365 american english idioms pdf