NETGEAR ReadyNAS - Replacement OS?

A few years ago, I got a NETGEAR ReadyNAS NV+ v2. I currently have 2x 4TB drives in there (WD Red) and I've been generally happy with the set-and-forget nature of the beast.

But these days I'm feeling that the OS is getting old, that it doesn't do what a modern NAS should be capable of. It's a bit finickity to administer and I'm wondering what my options are. I don't want to replace the entire NAS hardware ($$$) because it still works, and it only has a low demand placed on it. I use it for holding overflow photos from my laptop, for backing up my laptop, and storing videos for my OpenELEC media server. I would be interested in installing on it a media server (Plex?) (Media would be consumed on family iPhones/iPads) plus perhaps a P2P and usenet client on it. Perhaps a Squeezebox server might be handy too, but I am starting to experiment with (the free version of) MS Groove, having signed up to a family O365 subscription and placing my music etc. there.

Is there a suitable NAS OS (GPL or otherwise free/open source) that I could consider replacing the Radiator OS with? Has anyone tried this and what are your experiences? What other options are available to me?

Thanks

Jon