ReadyNAS Ultra 4 is slowing down my entire network LAN & WAN

Of course there's no easy way to tell what the hack was doing. But given the amount of traffic, you should assume the your files were being uploaded, and take whatever precautions are needed for identity theft. I'd back up the NAS, zero the disks, and do a clean install (upgrading to 4.2.27 T5). Then restore the data from backup. It isn't clear that an OS reinstall is enough (though it might be).

If that simply isn't practical, you should still back up the NAS before you attempt the OS reinstall, since the reinstall might not be able to fully resolve the hack (and it is conceivable that you might end up bricked as a result).

On the services - not sure why the router is blocking services by default (my R6300v2 is not), but programs don't generally get exclusive rights to internet ports. A few older and very common programs (web browsers, ftp, email) are allocated their own ports by the internet authority people. But not "age of empires" and probably not AIM - they use ports, but don't "own" them. That long list of services was reverse engineered by Netgear, and is mostly a convenience.

So internet ports are often used for multiple services, and there might be other reasons to block those ports that are not apparent in the service name. In any event, blocking ports that aren't supposed to be used is a conservative choice - the catch is that it is often difficult to know what ports are used by the applications you install.