How do make a link open in a specific Safari window in macOS?
As a developer I researched the possibility of extensions being able to achieve what you want by always opening a new window when clicking a link contained outside of Safari itself. I'm sorry to say I don't think this approach is going to be possible.

Unfortunately, Safari does not give extensions a way to determine the origin of a new tab/window. What I mean by this is that a new tab can be created because the user clicks a link in an email, command-clicks a link in Safari, opens a Safari bookmark while holding down Command, clicks a link within a PDF document, or any one of several other actions that will open a page in a new tab. Regardless of the method, Safari does not provide how the window was opened to a Safari extension.

Since all these events look the same, no extension can force links from outside Safari to open in new windows without also affecting links opened in other ways (including while browsing within Safari).