How to markup the documentation of the different version of our product?

considering that

 - The content is not strictly identical across version,
 - but it is very often the same, 
 - we wish newer version to be preferred by seo engines,
 - but older versions are preferred since they are older.



for a couple of year our strategy has been to follow the answers [to this question][1]

 - have all url prefixed by their version 
 - create a "latest" prefix that is a symlink pointing to the most recent version
 - the most recent version have a `<link rel="canonical"` targeting the "latest"
 - all version (except the most recent) start with a disclaimer indicating that this version is not the latest, this disclaimer include a link pointing to the latest version 



This is an example of our urls : 

 - old version : https://www.itophub.io/wiki/page?id=2_4_0%3Auser%3Astart
 - latest version : https://www.itophub.io/wiki/page?id=latest%3Auser%3Astart
 - latest version duplicate, with the `<link rel="canonical"` : https://www.itophub.io/wiki/page?id=2_7_0%3Auser%3Astart


This is an example of the latest, this disclaimer include a link pointing to the latest version 
[![picture of the link pointing to the latest version][2]][2]

-------------------------
This was meant to strengthen the "latest" url by having a stable url over time and by having several pages linking to it.

This does not seem to work, since google continues to prefer olds pages.
What should we do? 
I only see the nuclear weapon here :  adding on all version a `<link rel="canonical"` targeting the "latest".


  [1]: https://webmasters.stackexchange.com/questions/99867/how-to-correctly-mark-up-different-versions-of-the-same-document-which-are-non-c
  [2]: https://i.sstatic.net/kW5QO.png