Math thinking cannot occur without remembered math facts and skills. This fundamental truth cannot be denied. Although much despised by the education establishment, memorization is a powerful and necessary tool. Constantly encourage students to practice their developing math knowledge. Repetition fixes knowledge in memory.
Instant recall is essential for basic math facts.
They have to achieve automatic use of such facts. Then their minds will be free to focus on the next level of math knowledge. Encourage students to speak and write, using mathematically correct language and symbols. Let them write, cross-out, revise, and evolve their thinking on paper. Expect instant recall for basic math facts only.