Missing space on RN316 with 5*10TB discs

Yout help is very much appreciated!

Few follow-up questions:

1. How/why did a balance get "paused"? I am not aware of any way a user can do this?
2. When I inserted a new disk, there was nothing on the LED display, or on the web interface, indicating that any maintenance task is running or paused. Maybe this should be indicated somehow to prevent sittuations like the one in OP?
3. Cleaning up root volume will be hard to do without removing applications. I did a complete analisys and there is nothing in there that should not be. Is there a way to increase size of root fs?
4. On the sidenote, 4G size for root is probably not optimal in year 2018. And so is the fixed size of 1GiB for swap partition (especially since BTRFS does not support swap files)
5. Cannot find file named "smart_history.log"?
6. No warning was emailed or visible regarding any SMART issues?
7. Looking at full SMART reports in Webmin, all I see are 2 drives with few relocated sectors (248 sectors beeing the higest) which I would probably not classify as a "failing" drive... few timeouts too, but those where probably due to the high load at the time or because of relocating sectors. No pending sectors that I can see, so all relocated sectors are well within the reserved relocation space. Did I miss something?
8. On a sidenote - with 10TB and higer capacity of drives being comon these days, it is statistically unlikely a drive will go trough it's expected lifespan with zero relocated sectors. IMHO
9. How do I find out which drive is having isses? Do you beleive it needs to be replaced? Would you expect a manufacturer to accept that drive as failed?

Thanks you again for yout help!

Andrej