I could be wrong, but as far as I know one purchase (device or separate licence) usually qualifies for the current version and the next major version. That usually covers a couple of years.
If you have version 6.x or 7.x that probably will work with the next MacOS versions. I've got version 6 running on a Big Sur MacPro workstation. My licence qualifies for version 7 but the monitoring app doesn't work on my Intel Mac for some odd reason, so had to stay at version 6. I don't have experience with Mx Mac's and SoftRAID.
You may also look at Synology. Over the years I found their software a better fit for Mac users. A buddy of mine had a QNAP as well, but moved to Synology within a year due to problems connecting with his Mac's back then.
Another friend has a small business and we installed an 8-bay Synology unit around 10 years ago. It still is supported with the latest software and runs very fast. It is used for archiving, backups and also runs a couple of websites.
Currently I'm setting up a new 8-bay rack unit in my own network. It replaces a Drobo-S unit that has served me well for almost 12 years now.🙂
What I remember is the original ThunderBay IV with no drives @ $349.00 I bought in 2014 came with a 3 year warranty and as long as the buyer was within the warranty period they would receive free SoftRaid updates and upgrades. The last two TB enclosures I bought came with a one year warranty:
2019 - ThunderBay 4 w/Thunderbolt 3, no drives, and SoftRAID Lite XT @ $399.99
2021 - ThunderBay 4 w/Thunderbolt 3, no drives, and no software @ $369.99
Now they are charging $99.99 for an annual support plan to maintain the status quo. Keeping in mind that these enclosures are just metal boxes with a board and aren't NASs
and that changes to the kernel happen fairly rarely this is a lot of money!
I regret not buying a NAS. I run one RAID 5, the other is JBOD, and the original is just sitting on the shelf.
What else, ohh the TB cable connections are horrible if you touch the cable the drive disconnects. I had to pay for the little blue 'glue on connectors' and a
Drill America m1.8 x .35 Tap and 1.45mm Drill Bit Kit to fix this design issue on the 2014 and 2019 enclosures. The 2021 has pre-installed screw holes for the connectors. I also had to replace the fans with Noctua NF-A9 FLX because the original fans are too friggen loud.
Maintaining these things is getting to be
too expensive for very little in return.
The other option is to just get a MacMini for a server and just don't ever upgrade the OS on it.