Wish there were straightforward answers to your question. Let's see how best we can go about it.

Different stages of fracture healing need different level of mechanical stability as well as stimulation.

During fracture healing, the initial stages involve cellular proliferation. The predominant stimulus for proliferation is perfusion. Mechanical stimulation affects cell differentiation and depends on the strain magnitude and the cell phenotype. As a consequence, today's implants should be applied in a fashion that supports maximum perfusion at the fracture site. In the early period, the osteosynthesis should facilitate micro-motion of the fragments if secondary fracture healing is desired. At the same time, joint congruency, and axial and rotational positions have to be maintained. In the final period of healing, motion within the calcifying callus should be limited, which is naturally achieved by the increasing stiffness of the callus ossification.

You may find the following articles useful:

1. https://www.jstage.jst.go.jp/article/jnms/71/4/71_4_252/_pdf

2. Article Effect of mechanical stability on fracture healing—An update

The complexity of the topic may be judged from the fact that there are PhD thesis being done on it. One of them is

https://deepblue.lib.umich.edu/handle/2027.42/61660

Hope this helps.

Perfect answer

Micromotion has always been considered to promote healing at fracture site. Too much stiff implant construct in simple fracture pattern results in non or delayed union. However secondary healing when desired through application of biological plates is different scenario. Too much motion also hampers union as per Perren's strain theory.