Upgrading my RN104 from 6.1.9

I run all the maintenance checks quarterly (which defrags and balances less frequently than mdgm's suggestion).  I also leave bit-rot protection on (even though I am not running RAID).  Scrub acts as a good disk diagnostic (reading and verifying all files), so I think that is worth doing - even if errors can't be fixed, I want to know they are happening, so I can restore from backup.

For snapshots, the first question is whether they are appropriate for the share in the first place.  If the files are frequently modified (a torrent folder, or an sql database, ...) then you are better off turning snapshots off altogether.   In general my shares don't have a lot of files like that, so I leave snapshots on,

While Netgear does "thin" the snapshots over time, they won't deletely the automatic monthly ones.  IMO that's a huge mistake on their part - they should have a retention setting.  Once a month I go in, and delete the oldest snapshot in each share - keeping the oldest snapshot at 3 months old.  Your idea of the right threshold might be different, but I do suggest you go in monthly and delete snapshots older than your threshold.  

Start by figuring out how far back seems reasonable/useful to you.  If the space is excessive, then you might need to adjust your threshold, but treat that as a second step.  6.4.x gives more useful information on snapshot use than what you have now.