Looking at your image again, I think you should not use any detection model.
Your patterns are so tiny, they are more “spots” than patterns.
Instead I propose to create a small set of spot detection kernels and run them via convolution over the image.
A typical spot detection kernel would be a matrix like this:
0,  0, -1, 0, 0
0, -1, 4, -1, 0
-1, 4, 9, 4, -1
0, -1, 4, -1, 0
0, 0, -1, 0, 0
This example kernel should spike when it finds a spot with 2-3 pixels in diameter.
You would need kernels for different spot sizes…
A laplace filter from OpenCV could do the same trick…