A little observation. In the WebUI the free space continues to appear low even though there is much more in, when inserted an extra usb or sd card device for that and configured as /overlay.
In my case the 2 AR750s indicate that they have 2% free in WebUI when it is actually 97% free as it appears in Luci. USB devices are 480Mb.
I know it is only an aesthetic problem.
I think it’s because WebUI is looking at mtdblock6.
EDIT: Firmware 3.201 & 3.203, where AR750 is without space after updating, just 2Mb free.