I don't use niri but I worked around this problem (feature?) by creating a bash script that by default checks if a terminal is already open and if so, brings it into focus. Then I attach it to my default shortcut to open terminal and then create one more shortcut that opens a new terminal every time. So now, depending on which shortcut is pressed, I can either keep reusing the existing terminal or open a new one. I'm sure we can have a script that can do more fancy logic like allowing new terminals upto a given number and after that just bring the latest one into focus. Plenty of possibilities.