You only need to cover the last 2 years with your correspondence documents.
You need 1 document in each name, dated every 4 months from January 2018 to January 2020, and you need at least 3 different sources.
So, your documents should be dated in the following months:
January 2018
May 2018
September 2018
January 2019
May 2019
September 2019
Optional extra: January 2020
If you cannot meet those months, you could shift it by one month either way.
For example:
December 2017
April 2018
August 2018
December 2018
April 2019
August 2019
December 2019 (optional)
Or:
February 2018
June 2018
October 2018
February 2019
June 2019
October 2019
February 2020 (optional)