I think you should pull up some boards to eliminate the break line. Extra work and cost but the break will look pretty bad. Also, no matter how you do it, it's going to be very difficult to get a clean edge cutting installed HW flooring. A router might work if you use some double sided tape to hold a straight edge and make cuts that eliminate the break line. Clean up corners with a small chisel, etc.
“The single biggest problem in communication is the illusion that it has taken place.”
George Bernard Shaw
Redondo Beach, CA