kamień milowy po angielsku:

to reach a milestone

We started by setting out milestones.

Jason had never mixed with other children before, so his first day at school was a milestone in his life.

This voyage MARKED an important milestone in the history of exploration.

a stepping stone to something

an event or experience that helps you achieve sth else [+ to]: I see this job just as a stepping stone to better things. / 2. This could be a stepping stone, you know. / 3. Just think of it as a stepping stone.