How to create a Windows 10 bootable USB on a Mac for PC without Bootcamp, when install.wim is too large
The open-source package called wimlib, can also &quot;optimize&quot; a wim file.
I managed to reduce the size bellow 4Go, my FAT32 usb thumb accepted it, and booted with success
Install wimlib via brew
brew install wimlib
Copie the bigfile from the iso volume to your HD Disk (Download example by example)
cp /Volumes/CCC.../sources/install.wim install.wim
Compress the file
wimlib-imagex optimize install.wim --solid
&quot;install.wim&quot; original size: 4463411 KiB 
Using LZMS compression with 8 threads 
Archiving file data: 9 GiB of 9 GiB (100%) done 
&quot;install.wim&quot; optimized size: 3311533 KiB 
Space saved: 1151878 KiB

You may need sudo, because you could encounter an error like this:
[ERROR] Can't modify &quot;/home/roger/win/sources/install.wim&quot;: Permission denied 
ERROR: Exiting with error code 71: 
The WIM is read-only (file permissions, header flag, or split WIM).

Copy the reduced file to your usb drive
cp install.wim /Volumes/MYUSB/sources/install.wim
Boot up !