This is on my device with wifi-qcom-ac and container packages + large configuration with many scripts:
free-hdd-space: 360.0KiB
total-hdd-space: 16.0MiB
free space is stable, but I’m using additional thumb drive attached to USB for other files, nothing is saved on flash from my side.
Regarding ROS v7 stability, since I’m using new wifi drivers (wifi-qcom-ac) occasionally after startup I have same issue described here: http://forum.mikrotik.com/t/v7-16-2-stable-is-released/178911/1 and I need to reboot again if it happens, never experienced twice in a row after reboot but still I hope it will get fixed.
Regarding config migration from v6 to v7, you will probably need to change something, depends on configuration, never done it to tell it more. I know for sure you need to set up wifi from scratch when switching from legacy to new wifi drivers, but this is not mandatory since legacy drivers (wireless package) are still working on v7.