Which apps are a must have for your NAS?

Linux-dash is probably the best for overall additional monitoring.  For anything else, it all depends on what you want your NAS to do, with the processing power of your particular NAS being a potential issue.  The lower powered CPU and less RAM the NAS has, the fewer apps you want to run.

I run very few apps, though I have a powerful rack-mount NAS.  I have OwnCloud on an OS6 converted Pro2 (that mounts some shares from my primary NAS as read-only), and my main one runs DDClient and Zerotier.  Both also run Eaton Intellegent Power Protector (IPP) and HDSentinel, which I installed via SSH.  I use Zerotier instead of ReadyCloud for remote access (including remote backup, which ReadyCloud does not include) and DDClient because I need it for OwnCloud and remote access to my surveillance system and just never moved it to the Pro2 or surveillance system computer.  Eaton IPP is unique to Eaton UPS's, providing a lot more info than the bult-in NUT, and I like the way i can have HDSentinel create drive summaries for my main and backup NAS, with the ones from the other NAS copied to the main one for review even when they are powered down.

I don't use Plex or ReadyDLNA because my media box can access media via SMB or NFS and I don't store anything that's going to need transcoding to watch/listen to.

Unfortunately, some of the apps are becoming a little long in the tooth.  From what I have seen here, Netgear's support to app developers is abysmal, so I'm not surprised.  Right now, for example, they cannot build anything for OS6.10.3 on an ARM system because Netgear "left out" required items from the development package.  And similar things have happened before.

Note that there are also apps that are not available through the official Netgear path.  Some are on GitHub (like ZeroTier) and WhoCares  https://rnxtras.com/  is also very popular.  He does charge a minimal fee to help offset keeping his site up, but it's quite reasonable and he does a bang-up job of keeping his apps up to date (when Netgear doesn't throw down obstacles).  My  OwnCloud installatrion is his version.