I have been using my own snapshots of OpenWrt on the GL-MT6000 but I have always used the sysupgrade image to retain the OEM bootloader. My question is: if I build with CONFIG_TARGET_ROOTFS_PARTSIZE=512 and then install the factory image, will /dev/mmcblk0p7 change to reflect my 512 MiB definition thus leaving the remaining space free?
My goal is to adjust the rootfs size on this device down from 7.2G to something sane like 0.5G. See this post.
I am also considering @hnyman's post here which warns about just flashing factory directly. He thinks there are extra steps involved.