Scrub, Defrag, Balance: Recommended order ro remedy severely neglected maintenance

ReadyNAS 312

Firmware 6.9.2

2x4TB hard drives

~3.5TB of usable space. 

~2TB of data 

Should leave me ~1.5TB of free space.  However, I am at less than 100MB of free space. 

I've read through a bunch of posts and checked the usual culprits regarding hidden files and old snapshots (I am not running snapshots).  I just now figured out that this thing has never had a scrub, defrag, or balance done.....BAD ON ME!!!!

I've seen a couple posts where it was recommended to do maintenance in the following order:  scrub, defrag, balance.  I took that to be "regularly scheduled maintenance".   In my case, I am doing a massive cleaning of a system that has been woefully neglected and want to see if anybody recommends I change the order for this initial shot of TLC for this system. 

Also, can I use the system at all when this is going on or should I make sure it does not get used when this is taking place?  I'm trying to do it over the weekend when the business is not open but if it is still working through the process(es) come Monday morning when they need to use it, I need to know whether to stop the process or just let them know performance may be a little degraded.

Thanks in advance for any advice......