Hello There,
Thanks for providing the mockup of what you want to do. I would say that using the flex layout with your design will only lead to a more complex issue relating to the responsiveness and the other elements inside that columns.
If I were to designed that using the Pro editor, I would have one section with 7 rows. 1 row with only one column and the rest will have two custom columns (1/3 + 2/3).
For my first row, I would place the one column row as the container of the headline.
The rest of the rows will have a custom column settings having to have 1/3 + 2/3 setting. I will be placing the date headline on the left side and the text description on the right side columns.
And one last thing, I will create the vertical light with the vertical marker in an image editor like photoshop and make it as a background image for my rows so that it will mimic the timeline links between the rows and columns just like in your mock up.
Hope this make sense.