Rented an oscillating floor sander and went to town.
After sanding with 36 grit, 60 grit, and 80 grit it was time to fill the floor gaps. I mixed up the sanding dust from the 80 grit pass with wood flour cement. The mixture was then spread all over the floor and left to dry. I worked in small batches because of how quickly the mixture dried, about 10 minutes.
After filling the floor gaps one last final sanding of 80 grit was done.