DeX: file copy?

I came across with a very interesting problem: Given a Samsung phone (Fold3), and an USB docking station, and 2 thumb drives. I wanted to copy files (10Gb total) from Thumb1 to Thumb2 with the phone (FX commander). No matter what I tried, 10 of 10 attempts failed with various reasons (USB became instable, please reset phone, etc etc etc). Sometimes the whole UI was frozen for a couple of minutes. Then I realized that using all the same equipment, but also plugging a monitor to the dock solves the file copy issue. If I do this on DeX, 10 of 10 times success. What could I do to make this work without DeX?