For $\theta\in[0, \dfrac{\pi}{2}]$, the following inequality $\sqrt{2}(2a+3)\cos(\theta-\dfrac{\pi}{4})+\dfrac{6}{\sin\theta+\cos\theta}-2\sin2\theta<3a+6$ is always true. 
Determine the range of $a$.
1. **Substitute and Simplify**: 
   Given the inequality:
   \[
   \sqrt{2}(2a+3)\cos\left(\theta - \frac{\pi}{4}\right) + \frac{6}{\sin\theta + \cos\theta} - 2\sin 2\theta < 3a + 6
   \]
   Let \( t = \sin\theta + \cos\theta \). We know that:
   \[
   t = \sqrt{2}\sin\left(\theta + \frac{\pi}{4}\right) \quad \text{and} \quad t \in [1, \sqrt{2}]
   \]
   Also, we have:
   \[
   \sin 2\theta = 2\sin\theta \cos\theta = t^2 - 1
   \]

2. **Transform the Inequality**:
   Substitute \( t \) and \(\sin 2\theta\) into the inequality:
   \[
   \sqrt{2}(2a+3)\cos\left(\theta - \frac{\pi}{4}\right) + \frac{6}{t} - 2(t^2 - 1) < 3a + 6
   \]
   Since \(\cos\left(\theta - \frac{\pi}{4}\right) = \frac{t}{\sqrt{2}}\), the inequality becomes:
   \[
   (2a+3)t + \frac{6}{t} - 2t^2 + 2 < 3a + 6
   \]

3. **Rearrange the Inequality**:
   Rearrange the terms to isolate \(a\):
   \[
   (2a+3)t + \frac{6}{t} - 2t^2 + 2 < 3a + 6
   \]
   \[
   2at + 3t + \frac{6}{t} - 2t^2 + 2 < 3a + 6
   \]
   \[
   2at + 3t + \frac{6}{t} - 2t^2 + 2 - 3a - 6 < 0
   \]
   \[
   2at - 3a + 3t + \frac{6}{t} - 2t^2 - 4 < 0
   \]
   Factor out \(a\):
   \[
   a(2t - 3) + 3t + \frac{6}{t} - 2t^2 - 4 < 0
   \]

4. **Analyze the Expression**:
   Since \(2t - 3 \leq 2\sqrt{2} - 3 < 0\) for \(t \in [1, \sqrt{2}]\), we need:
   \[
   a > \frac{2t^2 - 3t - \frac{6}{t} + 4}{2t - 3}
   \]
   Define the function:
   \[
   f(t) = t + \frac{2}{t}
   \]
   We need to find the maximum value of \(f(t)\) in the interval \(t \in [1, \sqrt{2}]\).

5. **Find the Maximum Value**:
   Evaluate \(f(t)\) at the endpoints:
   \[
   f(1) = 1 + \frac{2}{1} = 3
   \]
   \[
   f(\sqrt{2}) = \sqrt{2} + \frac{2}{\sqrt{2}} = \sqrt{2} + \sqrt{2} = 2\sqrt{2} \approx 2.828
   \]
   The maximum value of \(f(t)\) in the interval \([1, \sqrt{2}]\) is 3.

6. **Conclusion**:
   Therefore, the inequality holds for:
   \[
   a > 3
   \]

The final answer is \( \boxed{ a > 3 } \)