Lets check that we have the correct nomenclature here. There is exFat and vFat files systems on the OpenWrt side and EXfat and FAT32 on the Windows side.
I know that micro-SD cards 64GB and above are coming delivered with EXfat, which is a Microsoft proprietary format. I am not aware that any other device is formatted as such, at least by default, and honestly not clear on what advantages EXfat has if any.
I use FAT32 with kmod-fs-vfat for most of my devices as I run Windoze and suck up the 2GB FS limitation. I have just checked my Kingston MLWG2 and it has no issue booting the SD Card formatted FAT32.
My global config section is identical.
My mount sections specify the uuid and file system, but I configure the devices thru the Luci interface and mount points does the heavy lifting. I also have Samba installed and manage permissions thru that.
May pay to list all the packages you installed for USB.
If you truly are running EXFAT, then you may wish to see this post. I did try to use exFat and just gave up.
https://forum.openwrt.org/viewtopic.php?id=65745
https://forum.openwrt.org/viewtopic.php?id=64608
There are a few other refs to EXFAT if you search the forum