Still no solution, but just today, analyzing another bcm27xx sysupgrade problem, I noticed that the regular upgrade function, default_do_upgrade() in package/base-files/files/lib/upgrade/common.sh, does
sync
echo 3 > /proc/sys/vm/drop_caches
The bcm27xx specific implementation, platform_do_upgrade() in target/linux/bcm27xx/base-files/lib/upgrade/platform.sh, does not have that.
The kernel docs say that this includes flushing cached inodes - could it be that not flushing the cache is what keeps f2fs partition busy?
Would it make sense to add echo 3 > /proc/sys/vm/drop_caches to bcm27xx platform_do_upgrade()?