SQL will use a proportional fill algorithm across the data files (in the same filegroup) to try and keep the amount of free space in each file the same. If your original file is still the hotspot this suggests this file is a lot emptier.

Do you have the space to increase the size of the newer files? Perhaps do that and shrink the original to remove some of the free space, then rebuild your indexes?

You will need an outage anyway so no need to move all the data out first (just take a backup)