I would like to use monospaced font for unknown file types and a specific font only for .txt files if possible.

That’s not the way that Notepad++ works. All unknown file extensions are interpreted by Notepad++ as “Normal Text”, and all Normal Text, whether it ends in .txt or not, get the Default Style applied. That’s the way Notepad++ was designed.

You might be able to get your desired behavior by creating a User Defined Language (UDL) associated with the .txt extension, and use the UDL dialog to set the font for just .txt. (This would override .txt so it’s using the UDL lexer, and is no longer considered “Normal Text”)

• official UDL overview: https://npp-user-manual.org/docs/user-defined-language-system/
• UDL details: https://ivan-radic.github.io/udl-documentation/

Style Configurator > Global Styles > Default Style is the style used for normal text (.txt) or any unknown file-types.