RAIDiator or OS6

RAIDiator is limited to SMB1 and TLS1.1.  Both of these have been depreciated, and current work-arounds in the OS's of the devices you use to access the NAS may go away.  OS6 supports SMB3 and TLS1.3.  So "OS6 is newer and has updates and support" is perhaps a lot more important that you seem to think, depending on the protocols and OS's you use to access it.  Since it's impossible to administer a ReadyNAS without the GUI, which relies on TLS, that may be the primary driver.  You can head that possibility off by making the change when it's your choice rather than if/when suddently forced to do so.  There are security holes in these protocols and other places within the outdated Linux distro that RAIDiator is based on, but a typical home user who does not intentionally expose the NAS to outside access and has reasonable virus and malware protection in place on other networked devices probably has little to fear.

Then again, that OS6 has "available support" is not the entire story, as Netgear will not provide support for a legacy NAS converted to OS6, even paid.  This forum obviously does support it.  So, your potential need for Netgear support can be an overwhelming part of the decision.

When you say you use it "without RAID", are all the drives independent volumes?  If so, then the volume expansion capability isn't a factor.  But unless your storage is only of backup data (there is another copy elsewhere) or you have secondary backup, you are putting your data at risk doing that.  OS6 does give you new options.

Another big advantage of OS6 that you did not mention is that the drives from a converted Pro Pioneer could be put directly into a 6 or more bay native OS6 NAS and "just work".  If your Pro dies and cannot be resurrected, there is no need to figure out how to recover the data (if you would need to based on your use) and move it to a new NAS.  So, again, you make the migration at your convenience rather than when forced to do so.

I personally moved my devices to OS6 a few years ago after the initial issues like fan control were fixed.  I have not regretted my decision.  But, my use sounds different from yours.  I pretty much store nothing on the local hard drives of my computers -- everything goes on the NAS and I have other NAS as backup.  I had also already upgraded the processor on my ProBE units and the RAM on all units, so I cannot comment on how a Pro Pioneer would run with OS6 and the original processor.  Upgrading to at least 2GB of RAM is a recommendation we now make for running OS6 at all.

Long story short:  It's still not a slam-dunk for every case.  But I believe the case for conversion is being made better the more time goes on.

Then, there is the possibility of conversion to a more generic Linux, which is a whole other discussion.