AFAIK you simply cant do what you are trying to do using "Similar series"
You have to use something as an intermediate step. One way of doing this would be to use a user category.
Create a new user category in the tag browser.
Do a search for the tag - that gives you the first book in each series.
Select all the series, then using the User Category plugin right-click and add the selected series to your user category.
Now you can click on the user category in the tag browser to see all those books.